Nanostructures, their use and process for their production

ABSTRACT

A lubricating and shock absorbing materials are described, which are based on nanoparticles having the formula A 1-x -B x -chalcogenide. Processes for their manufacture are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase Application of PCT International Application No. PCT/IL2011/000228, International Filing Date Mar. 10, 2011, which is a Continuation in part of U.S. patent application Ser. No. 12/721,113, filed Mar. 10, 2010, which are hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to inorganic nanoparticles for use as superlubricants and shock absorbing materials and methods of manufacturing such particles.

BACKGROUND OF THE INVENTION

MoS₂ and WS₂ are quasi two dimensional (2D) compounds. Atoms within a layer are bound by strong covalent forces, while individual layers are held together by van der Waals (vdW) interactions. The stacking sequence of the layers can lead to the formation of a hexagonal polymorph with two layers in the unit cell (2H), rhombohedral with three layers (3R), or trigonal with one layer (1T). The weak interlayer vdW interactions offer the possibility of introducing foreign atoms or molecules between the layers via intercalation. Furthermore, MoS₂, WS₂ and a plethora of other 2D compounds are known to form closed cage structures which are referred to as inorganic fullerene-like (IF) and inorganic nanotubes (INT), analogous to structures formed from carbon [1]. One of the initial methods of synthesis of IF-MoS₂ and IF-WS₂ involved starting from the respective oxide nanoparticles [2, 3]. Subsequently synthesis of IF-NbS₂ and IF-MoS₂ using a gas-phase reaction starting from MoCl₅ and NbCl₅, respectively, and H₂S has been demonstrated [4a, 5]. A similar strategy for the synthesis of IF-MoS₂ nanoparticles using the gas phase reaction between Mo(CO)₆ and sulfur, has been reported [4b]. The two kinds of reactions progress along very different paths, which have a large effect on the topology of the closed-cage nanoparticles. The conversion of the metal-oxide nanoparticles to sulfides starts on the surface of the nanoparticles progressing gradually inwards in a slow diffusion-controlled fashion. Contrarily, the gas-phase reaction proceeds by a nucleation and growth mode starting from, a small, e.g. MoS₂, nuclei and progressing outwards rather rapidly.

Modification of the electronic properties of layered-type semiconductors can be accomplished by doping/alloying process of the semiconductor, in which metal atoms go into the semiconductor layer, substituting the host transition metal or the chalcogene atoms. If the substituting atom (e.g. Nb) has one less electron in its outer shell than the host metal atom (Mo), the lattice becomes p-doped. If the substituting metal atom has one extra electron (Re), the lattice becomes n-type. If the substituent atom is chlorine, replacing the sulfur atom, the nanoparticle becomes n-type. Doping is usually limited to below 1 at % substitution. In the case of alloying, the guest atoms are of significant concentrations (>1%), in which case if the percolation limit is surpassed (e.g. Mo_(0.75)Nb_(0.25)S₂), the lattice becomes essentially metallic.

Alloying or doping of inorganic nanotubes has been reported for specific cases of Ti-doped MoS₂ nanotubes, Nb-doped WS₂ nanotubes [6(a),(b)]. In addition, W-alloyed MoS₂ nanotubes have been synthesized by varying the W:Mo ratio [6(c)]. However, since there was not much control of the amount of foreign atoms in the lattice, control of the electronic properties of the nanoparticles could not be achieved in the previous works.

Control of the doping level in nanoparticles, and in particular in inorganic nanotubes and fullerene-like structures can lead to various unique phenomena. Addition of various nanoparticles to boost tribological performance (i.e., reduced friction and wear) of lubricating fluids has been under investigation for sometime [7]. Equally important is the need for replacing the current oil-additives, in the search for environmentally more benign counterparts. Using semiconducting nanoparticles, like MoS₂ and WS₂ or the respective selenides as additives to lubricating fluids provides a unique tool to understand the electronic component of friction and wear. Thus, there arises the need for a new synthetic strategy of using inorganic fullerene-like (IF) nanoparticles and inorganic nanotubes (INT) of semiconductors, doped with metal and non-metal atoms.

RELATED REFERENCES

-   [1] Tenne, R. Nature Nanotech. 2006, 1, 103. -   [2] Tenne, R., Margulis, L., Genut M. & Hodes, G. Nature 1992, 360,     444. -   [3] Feldman, Y., Wasserman, E., Srolovitz D. J. & Tenne R. Science     1995, 267, 222. -   [4] (a) Deepak, F. L.; Margolin, A.; Wiesel, I.; Bar-Sadan, M.;     Popovitz-Biro, R.; Tenne, R. Nano 2006, 1, 167.

(b) Etzkorn, J.; Therese, H. A.; Rocker, F.; Zink, N.; Kolb, Ute.; Tremel, W. Adv. Mater. 2005, 17, 2372.

-   [5] Schuffenhauer, C.; Popovitz-Biro R.; Tenne, R. J. Mater. Chem.     2002, 12, 1587. -   [6] (a) Zhu, Y. Q.; Hsu, W. K.; Terrones, M.; Firth, S.; Grobert,     N.; Clark, R. J. H.; Kroto H. W.; Walton, D. R. M. Chem. Commun.     2001, 121;

(b) Hsu, W. K.; Zhu, Y. Q.; Yao, N.; Firth, S.; Clark, R. J. H.; Kroto H. W.; Walton, D. R. M. Adv. Funct. Mater. 2001, 11, 69;

(c) Nath, M.; Mukhopadhyay, K.; Rao, C. N. R. Chem. Phys. Lett. 2002, 352, 163;

-   [7] Donnet, C.; Erdemir A. Surf. Coat. Tech. 2004, 180-181, 76-84. -   [8] Rapaport, L.; Bilik, Y.; Feldman Y.; Homyonfer, M.; Tenne, R.     Nature 1997, 387, 791-793. -   [9] Katz, A.; Redlich, M.; Rapaport, L.; Wagner, H. D.; Tenne, R.     Tribol. Lett. 2006, 21, 135-139. -   [10] Naffakh, M.; Martin, Z.; Fanegas, N.; Marco, C.; Jimenez, I. J.     Polym. Sci. B: Polym. Phys. 2007, 45, 2309-2321. -   [11] Seifert, G.; Terrones, H.; Terrones, M.; Jungnickel, G.;     Frauenheim, T. Phys. Rev. Lett. 2000, 85, 146-149. -   [12] Wildervanck, J. C.; Jellinek, F. J. Less-Common Metals 1971,     24, 73. -   [13] Marzik, J. V.; Kershaw, R.; Dwight, K.; Wold, A. J. Solid State     Chem. 1984, 51, 170. -   [14] Coleman, K. S.; Sloan, J.; Hanson, N. A.; Brown, G.; Clancy, G.     P.; Terrones, M.; Terrones, H.; Green M. L. H. J. Am. Chem. Soc.     2002, 124, 11580. -   [15] Brorson, M.; Hansen, T. W.; Jacobsen, C. J. H. J. Am. Chem.     Soc. 2002, 124, 11582. -   [16] Kopnov, F.; Yoffe, A.; Leitus, G.; Tenne, R. Phys. Stat. Solidi     B 2006, 243, 1229-1240 -   [17] Scheffer, L.; Rosentzveig, R.; Margolin, A.; Popovitz-Biro, R.;     Seifert, G.; Cohen, S. R.; Tenne, R. Phys. Chem. Chem. Phys. 2002,     4, 2095.

SUMMARY OF THE INVENTION

Doping of inorganic fullerene-like (IF) nanoparticles and/or inorganic nanotubes (INT), endows n-type or p-type conductivity to these nanoparticles and reduced agglomeration and sedimentation. Using such modified nanoparticles as additives to lubricating fluids, leads to a remarkable reduction in the friction coefficient, i.e. the ratio of the force of friction between two surfaces, of metal-metal interfaces (below 0.015) and their wear behavior under different lubrication conditions. When added to oils, it was found that free electrons produced by substitional dopant atoms are trapped by defects on the nanoparticle surface. The negatively charged nanoparticles, which are possibly surrounded by a positively-charged ionic atmosphere in the oil, repel each other at close proximity. This effect leads to smaller agglomerates than for the undoped nanoparticles and formation of stable oil suspension, even in the absence of a surfactant.

Furthermore, it was found that the doped nanoparticles are less susceptible to the electrostatic charges formed during tribological work. These observations suggest a fundamental shift from the present understanding of the electronic effects in tribological interfaces. It furthermore offers numerous applications for these nanoparticles.

The term dopant atoms or doping atoms refers to atoms which are different from the atoms comprising the nanoparticles. In some embodiments, the dopant is present in the nanoparticle in a concentration lower than 1 at %. In other embodiments, the dopant is present in the nanoparticle in a concentration lower than 30 at %.

Substitutional dopants are doping atoms which replace some of the atoms comprising the nanoparticles in specific positions of the crystalline lattice. Higher concentrations of dopants (e.g., >1 at %) may result in alloying and clustering phenomena leading to a poorly controlled electronic properties of the nanoparticles. These substitutional dopants are capable of providing additional electrons (or holes) to a host material, thereby creating an excess of negative (or positive in the case of holes) charge carriers, referred to as a state of n-type (or p-type) conductivity.

The term tribology refers to friction related phenomena, such as wear, accumulation of electrical charge at the interacting surfaces, etc. Multiwall closed-cage (fullerene-like) nanoparticles of WS₂ and MoS₂ and nanotubes were shown to exhibit very good tribological behavior with numerous potential applications, like additives to lubricating fluids [8], as self-lubricating coatings [9] and for improving the mechanical behavior of nanocomposites [10]. The present invention now provides the synthesis and formation of nanostructures of the general structural formula A_(1-x)-B_(x)-chalcogenides. The nanostructures of the present invention are inorganic nanotubes (INT) or mixtures thereof with other nanoparticles such as inorganic fullerene-like nanoparticles (IF). The invention concerns also a composition containing IF nanostructure and INT of the above formula. B is incorporated into the lattice of the A-chalcogenide as a doping atom, thereby altering its characteristics inter alia as a function of the nature of A, B and the amount of B, i.e. the value of x in the A_(1-x)-B_(x)-chalcogenide lattice. Similarly, halogen (CF etc) and pnictides (P³⁻, etc) atoms can replace the chalcogen atom in the lattice leading to n- and p-type conductivity, respectively. The doping of B_(x) in the lattice of the A-chalcogenide produces changes in the electronic properties leading to the formation of high conductivity semiconductors, which are capable of dissipating electrical charges formed during tribological conditions (e.g. tribocharging). This, in turn, reduced the tribological work at interfaces, leading to reduction in the friction coefficient and improved lubrication.

Thus, the present invention is directed to inorganic nanotubes (INT) of the formula A_(1-x)-B_(x)-chalcogenide, wherein A is either a metal/transition metal or an alloy of such metals/transition metals, B is a metal or transition metal, and x being ≦0.3 provided that: A≠B. In some embodiments, x is below 0.01, or below 0.005.

The compound A may be a metal or transition metal or an alloy of metals or transition metals selected from the following: Mo, W, Re, Ti, Zr, Hf, Nb, Ta, Pt, Ru, Rh, In, Ga, and alloys like W_(x)Mo_(1-x), etc.

The compound B is a metal or transition metal selected from the following: Si, Nb, Ta, W, Mo, Sc, Y, La, Hf, Ir, Mn, Ru, Re, Os, V, Au, Rh, Pd, Cr, Co, Fe, Ni. Within the nanostructure, B and/or B-chalcogenide are incorporated within the A_(1-x)-chalcogenide. The chalcogenide is selected from S, Se, Te.

The substitution of B in A may be continuous or alternate substitutions. Continuous substitution are spreads of A and B within each layer alternating randomly (e.g. (A)_(n)-(B)_(n), n>1). Depending on the concentration of incorporated B, it may replace a single A atom within A_(1-x)-chalcogenide matrix forming a structure of ( . . . A)n-B-(A)n-B . . . ). Alternate substitution means that A and B are alternately incorporated into the A_(1-x)-chalcogenide lattice ( . . . A-B-A-B . . . ). It should be noted that other modes of substitution of the B in the A-chalcogenide lattice are possible according to the invention. Since the A-chalcogenide has a layered structure, the substitution may be done randomly in the lattice or every 2, 3, 4, 5, 6, 7, 8, 9 or 10 layers.

The present invention is further directed to a process for the synthesis of the inorganic nanotubes (INT) of the general structural formula A_(1-x)-B_(x)-chalcogenides.

The INT nanoparticles of the invention may be prepared by a variety of processes, e.g., gas phase processes and solid phase processes. In an exemplary gas phase process, A-Y₁ and B-Y₂ compositions each in vapor phase, where Y₁ and Y₂ are independently halogens (i.e. may or may not be the same) selected from chlorine, bromine or iodine are employed. The A-Y₁ and B-Y₂ vapors are flown into a reaction chamber together with the aid of a forming gas containing a reducing agent and an inert carrier gas. In the chamber the combined gas stream meets in an opposite direction a flow of a chalcogene carrying gas (the reacting gas), thereby causing reduction of the A and B metals or transition metals followed by a reaction with the chalcogene, resulting in the formation of said nanostructures.

In some embodiments, the A-Y₁ and B-Y₂ compositions in the vapor phase are prepared by evaporating A-Y₁ and B-Y₂ compositions in a chamber separate from the reaction chamber. A common or separate evaporation chambers may be used for preparation of vapors of the A-Y₁ and B-Y₂ compositions.

In accordance with the present invention, the metal or transition metal A-chalcogenide precursor may be a semiconductor of a certain electrical conductivity. Upon the insertion of an appropriate B element, the resulting nanostructure produced from said precursor has a higher electrical conductivity and further higher lubricity. Thus, the invention provides for manufacture of a nanostructured electrical conductor, which may further be characterized as lubricant materials. In accordance with the present invention, the metal atom B substitutes the metal A in the original lattice. Generically, atom B may have one extra valence electron or may be deficient in one such electron compared to the original A metal atom leading to n-type (donor) and p-type (acceptor) conductivity.

Thus, the present invention is further directed to novel donor composition (electron conductors) formed by INT-nanostructures, e.g. Re doped INT-MoS₂ and INT-WS₂, and novel acceptors (hole conductors), e.g. Nb doped INT-MoS₂ and INT-WS₂ and Cl doped WS₂(MoSe₂)-electron conductors. Other possible donor or acceptors according to the present invention are InS doped with Si being p-type or GaSe, InSe doped with Zn or Cd being n-type conductors. In addition one may consider the substitution of the chalcogene atom in the nanoparticle lattice. For example Br substitution of Se would make InSe nanoparticles an n-type conductor and P doped GaS nanoparticles will become p-type conductor.

As indicated above, the invention also provides a composition comprising a plurality of the nanostructures, as disclosed herein. This may for example be a composition comprising MoS₂ nanoparticlers and nanotubes doped by Nb, Re; or a composition comprising WS₂ nanoparticles and nanotubes doped by Nb, Re. The compositions of the invention may be used as lubricants or as lubrication materials

Due to the above described tribological properties of the nanostructures of the present invention, the composition(s) may also be used as shock absorbing materials, e.g., in a shock absorber. Due to the electrical properties of the nanostructures of the present invention, the above composition(s) may also be used in sensor devices for chemical or electromechanical type sensing.

The invention thus further provides in another of its aspects a lubricating material or lubricant comprising nanostructures, e.g., inorganic nanotubes (INT) and/or fullerene-like (IF) nanostructure, each being of the formula A_(1-x)-B_(x)-chalcogenide, wherein A is either a metal/transition metal or an alloy of such metals/transition metals, B is a metal or transition metal, and x being ≦0.3 provided that: A≠B.

In some embodiments, x is below 0.01. In some other embodiments, x is below 0.005. In further embodiments, x is between 0.005 and 0.01.

The invention also provides a shock absorbing material comprising nanostructures, e.g., inorganic nanotubes (INT) and/or fullerene-like (IF) nanostructure, each being of the formula A_(1-x)-B_(x)-chalcogenide, wherein A is either a metal/transition metal or an alloy of such metals/transition metals, B is a metal or transition metal, and x being ≦0.3 provided that: A≠B.

In some embodiments, x is below 0.01. In some other embodiments, x is below 0.005. In further embodiments, x is between 0.005 and 0.01.

Thus, according to yet another aspect of the invention, there is provided a process for the manufacture of inorganic nanotubes (INT), each having the formula A_(1-x)-B_(x)-chalcogenide, wherein A is a metal or transition metal or an alloy of one metals or transition metals including at least one of the following: Mo, W, Re, Ti, Zr, Hf, Nb, Ta, Pt, Ru, Rh, In, Ga, W_(x)Mo_(1-x), and others. B is a metal or transition metal selected from the following: Si, Nb, Ta, W, Mo, Sc, Y, La, Hf, Ir, Mn, Ru, Re, Os, V, Au, Rh, Pd, Cr, Co, Fe, Ni; and x≦0.3 (in some embodiments, less than 0.01, or less than 0.005), provided that within said nanostructure A≠B; and having B and B-chalcogenide doped within the A_(1-x)-chalcogenide. In another embodiment AX1_(-x)Y_(x) where X is the chalcogene atom (S, Se, Te) and Y can be either halogen (Cl, Br, I) or pnictide (P, As, Sb) with x being lower than 0.01; the process comprising:

providing A-Y₁ and B-Y₂ compositions each in vapor phase, Y₁ and Y₂ being the same or different halogens selected from chlorine, bromine or iodine;

flowing said A-Y₁ and B-Y₂ vapors together with a reducing agent carrying forming gas into a reaction chamber where they meet an opposite direction flow of a chalcogene carrying gas, thereby causing reduction of the A and B metals or transition metals followed by a reaction with the chalcogene, resulting in the formation of said nanostructures.

A process for the manufacture of nanostructures, such as INT or IF nanoparticles, each having the formula A_(1-x)-B_(x)-chalcogenide wherein A is a metal or transition metal or an alloy of one metals or transition metals including at least one of the following: Mo, W, Re, Ti, Zr, Hf, Nb, Ta, Pt, Ru, Rh, In, Ga, and alloys like W_(x)Mo_(1-x), and others; B is a metal or transition metal selected from the following: Si, Nb, Ta, W, Mo, Sc, Y, La, Hf, Ir, Mn, Ru, Re, Os, V, Au, Rh, Pd, Cr, Co, Fe, Ni; and x≦0.3 (in some embodiments, less than 0.01, or less than 0.005), provided that within said nanostructure A≠B; and having B and B-chalcogenide doped within the A_(1-x)-chalcogenide; the process comprising:

providing A-oxide and B-oxide compositions, each in vapor phase, (or in the solid phase under conditions permitting phase transformation into the vapor phase); and

permitting said A-oxide and B-oxide vapors to flow together with a reducing agent carrying forming gas (such as H₂S) into a reaction chamber causing reduction of the A and B metals or transition metals and subsequent formation of said nanostructures.

The invention also provides a process for the manufacture of nanostructures, such as INT or IF nanoparticles, each having the formula A_(1-x)-B_(x)-chalcogenide wherein A is a metal or transition metal or an alloy of one metals or transition metals including at least one of the following: Mo, W, Re, Ti, Zr, Hf, Nb, Ta, Pt, Ru, Rh, In, Ga, and alloys like W_(x)Mo_(1-x), and others; B is a metal or transition metal selected from the following: Si, Nb, Ta, W, Mo, Sc, Y, La, Hf, Ir, Mn, Ru, Re, Os, V, Au, Rh, Pd, Cr, Co, Fe, Ni; and x≦0.3 (in some embodiments, less than 0.01, or less than 0.005), provided that within said nanostructure A≠B; and having B and B-chalcogenide doped within the A_(1-x)-chalcogenide; the process comprising:

providing A_(1-x)-B_(x)-oxide composition, in vapor phase, (or in the solid phase under conditions permitting phase transformation into the vapor phase); and

permitting said A_(1-x)-B_(x)-oxide vapors to flow together with a reducing agent carrying forming gas into a reaction chamber causing reduction of the A and B metals or transition metals in said A_(1-x)-B_(x)-oxide and subsequent formation of said nanostructures.

The invention also provides a process for the manufacture of nanostructures, such as INT or IF nanoparticles, each having the formula A_(1-x)-B_(x)-chalcogenide wherein A is a metal or transition metal or an alloy of one metals or transition metals including at least one of the following: Mo, W, Re, Ti, Zr, Hf, Nb, Ta, Pt, Ru, Rh, In, Ga, and alloys like W_(x)Mo_(1-x), and others; B is a metal or transition metal selected from the following: Si, Nb, Ta, W, Mo, Sc, Y, La, Hf, Ir, Mn, Ru, Re, Os, V, Au, Rh, Pd, Cr, Co, Fe, Ni; and x≦0.3 (in some embodiments, less than 0.01, or less than 0.005), provided that within said nanostructure A≠B; and having B and B-chalcogenide doped within the A_(1-x)-chalcogenide; the process comprising:

providing A-chalcogenide, in vapor phase, (or in the solid phase under conditions permitting phase transformation into the vapor phase); and

permitting said A-chalcogenide vapors to flow together with a transport agent (e.g., a halogen such as chlorine or iodine) and a dopant precursor B-Y (wherein Y may be either halogen e.g., Cl, Br, I or pnictide, e.g., P, As, Sb) in a reaction chamber under conditions preventing or minimizing back transport of atoms of said dopant, thereby causing doping of B into A-chalcogenide and formation of said nanostructures.

In some embodiments, said A-chalcogenide are nanoparticles selected from INT and IF, said nanoparticles being prepared by chemical vapor transport (CVT) method.

In some embodiments, said transport agent is iodine.

In further embodiments, the reaction chamber being a two zone chamber, comprising a high temperature zone and a zone of a lower temperature, wherein said dopant precursor is fed into the high temperature zone and said A-chalcogenide nanoparticles are fed into the zone of a lower temperature, as further disclosed herein.

In some embodiments of the processes of the invention, x is less than 0.01. In some embodiments, x is less than 0.005. In other embodiments, x is between 0.005 and 0.01.

The invention also provides nanoparticles, IF and/or INT, obtained by any one process according to the invention.

Further provided are nanoparticles, IF and/or INT, obtainable by any one process according to the invention.

In another aspect of the invention, there is provided the use of any nanoparticle obtained/obtainable by any one process of the invention, as disclosed herein.

The above method may be used for the formation of a nanostructured electrical conductor.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

FIGS. 1A-1B show three tables presenting, respectively, various metal precursors suitable for the synthesis of the doped IF nanoparticles; IF-nanoparticles with possible p-type or n-type dopants; and IF-nanoparticles with possible magnetic dopants/impurities.

FIGS. 2A-2B is an example of an apparatus suitable for the manufacture of IF-nanostructures of the present invention including a main reactor (FIG. 2A) and a separate auxiliary furnace (FIG. 2B).

FIG. 3 shows XRD pattern of (a) IF-MoS₂ and (b) IF-Mo_(1-x)Nb_(x)S₂ nanoparticles prepared at T₁=850° C. and T₂=900° C. (series-2). Standard diffraction patterns of 2H—MoS₂ (long lines) and 2H—NbS₂ (short lines) are also shown for comparison.

FIG. 4 shows a zoom-in look of (002) peaks from FIG. 2. Standard diffraction patterns of 2H—MoS₂ (long line) and 2H—NbS₂ (short line) are also shown for comparison.

FIGS. 5A-5D show (FIG. 5A) TEM image of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles prepared at T₁=800° C. and T₂=850° C. (series-1), (FIG. 5B) and (FIG. 5C) HRTEM images of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles prepared at T₁=850° C. and T₂=900° C. (series-2). (FIG. 5D) shows the EDS spectra of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticle in (FIG. 5C).

FIGS. 6A-D show (FIG. 6A) HRTEM image and (FIG. 6B) the corresponding EELS spectrum of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles prepared at T₁=850° C. and T₂=900° C. (series-2), (FIG. 6C) Exploded view of a portion of nanoparticle in (FIG. 6A) showing mismatch in the layers, (FIG. 6D) HRTEM of another IF nanoparticle showing defects/dislocations in the layers.

FIGS. 7A-7C show (FIG. 7A) HRTEM image and (FIG. 7B) the expanded view of a portion of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles prepared at T₁=850° C. and T₂=900° C. (series-2), (FIG. 7C) Line profile of the boxed are in (FIG. 7B) shows the interlayer spacing to be 6.4 Å.

FIGS. 8A-8D disclose images of elemental mapping by energy-filtered TEM (EFTEM) of IF-Mo_(1-x)Nb_(x)S₂ nanoparticle, with a thin surface oxide layer. (FIG. 8A) Zero-loss image; (FIG. 8B) Sulfur map measured around the S L_(2,3) edge (167-187 eV); (FIG. 8C) Niobium map measured around the Nb L₃ edge (2370-2470 eV); (FIG. 8D) Oxygen map measured around the O K edge (532-562 eV).

FIG. 9 exhibits XPS line shape analysis of the Nb 3d signal (with Gaussian-Lorenzian components). Reduced Nb (I and II) is believed to be within the nanoparticles. The oxidized Nb (III) appears on the particle surfaces.

FIGS. 10A-10C demonstrate electrically induced line shifts of the XPS lines in the CREM mode: (FIG. 10A) Nb(3d), (FIG. 10B) Mo (3d_(5/2)) in IF-Mo_(1-x)Nb_(x)S₂ nanoparticles, (FIG. 10C) Mo (3d_(5/2)) in IF-MoS₂ nanoparticles. In all panels, curve (I) corresponds to ‘eFG off’ (electron flood gun off) conditions and curve (II) refers to ‘eFG on’. Note that (panel FIG. 10A) the oxidized Nb exhibits a large shift while the reduced Nb signal is practically not shifted at all. Comparison of panels (FIG. 10B) and (FIG. 10C) demonstrates the effect of Nb substitution on the Mo line shift, indicating improved conductance in the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles as compared to the undoped IF-MoS₂ nanoparticles.

FIGS. 11A-11C exhibit AFM measurements carried out on an IF-Mo_(1-x)Nb_(x)S₂ nanoparticle (FIG. 11A) AFM image, (FIG. 11B) I/V curves carried out on the nanoparticle (dashed lines), (FIG. 11C) corresponding dI/dV vs V plots. In both FIG. 11B and FIG. 11C, the corresponding plots of the IF-MoS₂ nanoparticle are shown for comparison (solid lines).

FIGS. 12A to 12C show a schematic presentation of the substitutional patterns of the Nb atoms within the IF-Mo_(1-x)Nb_(x)S₂ lattice.

FIGS. 13A and 13B show an experimental setup used in other examples of the invention including a main reactor (FIG. 12A) and a separate auxiliary furnace (FIG. 12B).

FIGS. 14A-14C show images and EDS spectra of obtained IF-nanostructures; FIGS. 14A and 14B show HRTEM images of IF-Mo_(1-x)Re_(x)S₂ nanoparticles synthesized at 850° C.

FIG. 14C show the EDS spectra of the IF-Mo_(1-x)Re_(x)S₂ nanoparticles shown in FIG. 14B.

FIGS. 15A-15D show the experimental results: FIGS. 15A and 15B are, respectively, the TEM and the EDS spectra of the IF-Mo_(1-x)Re_(x)S₂ nanoparticles synthesized at T₁=800° C., FIG. 15C is the individual HRTEM image of the IF-Mo_(1-x)Re_(x)S₂ nanoparticles synthesized at T₁=900° C.; and FIG. 15D shows the HRTEM images of IF-W_(1-x)Re_(x)S₂ nanoparticle synthesized at T₁=900° C.

FIG. 16 shows an XRD pattern of IF-Mo_(1-x)Re_(x)S₂ nanoparticles prepared at (a) 850° C. and (b) 900° C. Standard diffraction patterns of 2H—MoS2 and 2H—ReS2 are also shown for comparison. The asterix (*) in the diffraction pattern correspond to the peak arising from the filter used for collecting the nanoparticles. Also are shown in pattern are the peak of the oxide of Re (ReO₃—#) and Mo (MoO₂—+) which are undesirable product of the non-optimized reaction.

FIG. 17 shows a schematic drawing of the structure of a fragment of Re-doped (14,14) MoS₂ nanotube.

FIGS. 18A-18D show the HRTEM images of IF-W_(1-x)Re_(x)S₂ nanoparticles synthesized at 900° C. FIG. 18A and FIG. 18B show HRTEM images of elongated and faceted nanoparticles, respectively; FIG. 18C shows the EDS spectra of the synthesized nanoparticles. FIG. 18D gives a “ruler” for elucidating the dimensions of the obtained nanoparticles.

FIG. 19A shows a modified quartz reactor used for the synthesis of Re:MoS₂ nanoparticles; FIG. 19B shows the auxiliary horizontal furnace used to prepare Re_(x)Mo_(1-x)O₃.

FIG. 20 shows average of rhenium concentration (at %) results in Re:MoS₂ nanoparticles, measured by time of flight secondary ion mass spectrometry (TOF-SIMS).

FIG. 21 shows average of rhenium concentration (at %) results in Re:MoS₂ nanoparticles, measured by inductive coupled plasma mass spectrometry (ICP-MS).

FIG. 22A shows a typical high resolution scanning electron microscope (HRSEM) image of Re:MoS₂ nanoparticles; FIG. 22B shows an high resolution transmission electron microscope image of Re:MoS₂ nanoparticles, with an interlayer spacing as shown by the line profile of Re:MoS₂ nanoparticle (0.627 nm) coincides with that of an undopes MoS₂ nanoparticle.

FIG. 23 shows XRD pattern of MoS₂ powder and Re-doped MoS₂ powder.

FIG. 24A is an optical microscope image through the bottom of an oil suspension containing Re-doped MoS₂ nanoparticles 1 day after the initial mixing; FIG. 24B is an optical microscope image through the bottom of an oil suspension containing undoped MoS₂ nanoparticles 1 day after the initial mixing.

FIG. 25A shows a SEM micrograph of Re (0.12 at %):MoS₂ after spread experiment; FIG. 25B is a SEM micrograph of undoped MoS₂ after spread experiment.

FIG. 26A shows the resistivity vs. temperature for different Re doped and undoped MoS₂ samples; FIG. 26B shows calculated activation energies (300 K) for the different Re-doped MoS₂ nanoparticles.

FIG. 27A shows the friction coefficient vs. time measured for different samples in PAO-4 oil; FIG. 27B shows the average surface roughness of the pin surface after the tribological measurements.

FIG. 28 shows a reactor used in other examples of the invention suitable for the manufacture of IF-nanostructures of the present invention

FIG. 29 shows a reactor used for annealing of the IF-nanostructures produced in the reactor depicted in FIG. 28.

FIG. 30A-B show images of Re-doped IF-MoS₂ nanoparticles synthesized in the reactors depicted in FIGS. 28 and 29. FIG. 30A shows a scanning electron microscope (SEM) of the formed nanoparticles; FIG. 30B shows transmission electron microscopy (TEM) of the formed nanoparticles.

FIG. 31 shows an auxiliary reactor to prepare a solid solution of a precursor for the manufacture of IF-nanostructures of the present invention.

FIG. 32 shows a reactor used in other examples of the invention suitable for the manufacture of IF-nanostructures of the present invention using volatile precursors.

FIG. 33 shows an auxiliary suspender for the volatile precursors used in the reactor of FIG. 32.

FIG. 34 shows a different reactor set-up for doping INT-WS₂ nanotubes in a quartz ampoule with ReCl₄ or NbCl₄.

FIG. 35 shows a TEM image of the doped nanotubes obtained in the reactor depicted in FIG. 34.

FIG. 36 shows a modified fluidized bed reactor for synthesis of IF and INT-doped materials.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention is aimed at providing inorganic nanotubes (INT) and their mixtures, e.g., with other INT or IF nanoparticles, wherein the INT nanoparticles of the invention are of the formula A_(1-x)-B_(x)-chalcogenide, where A is a metal or transition metal or is an alloy of metals or transition metals doped by another metal or transition metal B different from A, and x does not exceed 0.3, in some embodiments, does not exceed 0.01, or does not exceed 0.005. Depending on the chemical differences between A and B within a particular A_(1-x)-B_(x)-chalcogenide lattice, and in particular, the crystalline structure of A-chalcogenide vs. B or B-chalcogenide, the concentration of B, i.e. the value of x may vary. In case A-chalcogenide and B-chalcogenide crystallize in a similar habit, x values may be in the range of up to 0.25, or even larger. More particularly, values of up to 0.08 to 0.12, more particularly, 0.1 to 0.15, and more so below 0.01 were obtained in accordance with the present invention. In case A-chalcogenide and B-chalcogenide crystallize in a dissimilar lattice habit, x may have much lower values, in the range of less than about 0.05. In particular, x values in accordance with such a case may be up to 0.001 to 0.01 or 0.03.

Doping of INT enables use of Mo/WS₂ nanostructures for semiconducting nanoelectronic devices, such as transistors, energy generators (solar cells), nanosensors (e.g. in medical devices), and as a conductive phase in nanocomposites with numerous potential applications. Indeed, the band gap of MoS₂ nanotubes is similar to that of silicon: 0.89-1.07 eV, versus 1.17 eV for silicon. At the same time, S—Mo—S layers with coordinatively saturated surfaces are much more resistant against oxidation and humidity than silicon or other semiconductors of the groups IV, III-V, II-VI. A nanocomposite comprising nanostructures of the invention may be in the form of a matrix with the nanostructures embedded therein.

Re (Nb) doped INT-WS₂ and INT-MoS₂ and IF thereof exhibit improved tribological behavior. Since the doped nanoparticles are highly conductive, they tend to repel each other and disperse better in the suspension. Furthermore, they exhibit smaller tendency to entrain in the contact area, allowing them to move and role freely. These properties enable use of the nanoparticles of the present invention in lubricant compositions (nanocomposites with self-lubricating behavior, e.g. being fluids or films). For example, the nanostructures are embedded in a matrix (solid or liquid) in an appropriate amount or concentration to provide efficient lubrication. The (Re, Nb) doped nanoparticles may exhibit improved tribological behavior for two reasons. Their excess free carriers allow them to repel each other, thus minimizing their tendency to agglomerate. Furthermore, the nanoparticles can accumulate extra charge during the tribological action. This extra charge may lead to their rapid chemical deterioration. However, the doped nanoparticles are expected to more easily conduct this extra charge, making them chemically more stable than the undoped ones. It should, however, be noted that excess doping might lead to extra strain and imperfections, like edge or misfit dislocations, in the structure of the INT nanoparticles. It is therefore preferable to control the doping level to avoid excess doping, e.g. so as not exceed 1 at % or preferably be below 0.1 at % concentration.

It should be understood that the INT nanostructures of the present invention could be used in various applications, including conventional silicon-based applications, because of a possibility of n- or p-type doping of these nanostructures like for silicon.

Due to their hollow structure, the INT nanostructures of the present invention may further be used for absorbing kinetic energy for alleviating various kinds of shocks (like shocks occurring by car accidents) and interactions between moving particles. The composition of the present invention formed by the above-described nanostructures may thus be used as or within a so-called “shock absorber” device.

Many of the potential device applications envisaged for INTs according to the invention require high carrier density in order for them to easily transport charge and establish semiconducting (p-n or Schottky) junctions. This makes the p and n-doping of the nanotubes relevant to their future applications, e.g. as chemical or electromechanical sensor devices. However, excess doping may not only hamper the perfect crystallinity of the nanotubes, but may also lead to degeneracy, i.e. the Fermi level will go into the conduction (valence) band of the semiconductor nanotubes. In this case, they become semimetallic and would be not suitable for many electronic applications. Therefore, controlled doping according to the present invention enables to obtain engineered-like materials, where the amount of dopant is predetermined leading to nanostructures having improved electronic properties.

It should be noted that when the concentration of a dopant (i.e. component B in A_(1-x)B_(x)S₂) is not larger than at 1 at %, the B atoms are generally distributed as single atoms randomly in the lattice of the host. In this case, the properties of the A-chalcogenide lattice (like the energy gap) are preserved. Each guest atom (B) contributes an electron (donor) or hole (acceptor) and the carrier density of the host lattice is thereby modified. Under these conditions, the best control over the conductivity and tunability over the physical properties of the A-chalcogenide lattice is accomplished. When the concentration of the guest-B is larger than about 1%, clusters of B atoms and even regions of a sublattice-B-chalcogenide within the host lattice-A-chalcogenide are formed, in which case many of the physical properties of the lattice (like energy gap) are determined by the two sublattices according to some mixture rules. If the enthalpy of the two A-chalcogenide and B-chalcogenide compounds are very different, non-random distribution and even segregation of two distinguishable phases may occur in the lattice. The INT-nanostructure of the present invention are characterized by the best doping effects achieved by adding substantially not more than 1% of the guest. Some specific but not limiting examples of the novel composition of the present invention are INT-Mo_(1-x)Nb_(x)S₂ and INT-Mo(W)_(1-x)Re_(x)S₂, where Nb and Re respectively are incorporated (doped or alloyed) into Mo- and Mo- or W-chalcogenide. FIGS. 1A to 1B show two tables exemplifying, respectively, various metal precursors suitable for the synthesis of the doped INT nanoparticles and INT-nanoparticles with possible p-type or n-type dopants.

One example of such possible doped nanoparticle is Mo_(1-x)Re_(x)S₂. MoS₂ and ReS₂ crystals possess a layered structure. Each molecular layer is made of a S-M-S (M=Mo,Re) sandwich and the layers are stacked together by weak van der Waals interactions. While 2H—MoS₂ has trigonal prismatic structure (P6₃/mm) group [11], ReS₂ crystallizes in a triclinic unit cell (P-1 group) with Re—Re atom bonds forming a chain of interlinked parallelograms [12]. This structural disparity leads to very limited solubility of the Re atoms in 2H—MoS₂. Also, both MoS₂ and ReS₂ are semiconductors with bandgap of 1.3 and 1.1-1.3 eV, respectively [11,13]. These considerations suggest that overdosing the MoS₂ nanoparticles or the respective nanotubes with Re atoms may lead to segregation of ReS₂ islands. These considerations resulted in a careful study of the Re atom content and optimization (ca. 0.12 at %), which was achieved by gradually increasing the rhenium oxide content in the molybdenum oxide precursor.

As for the layered transition-metal dichalcogenide ReS₂, it is a diamagnetic semiconductor that possesses an indirect gap in the near-infra-red (NIR) region of about 1.37 eV. The layered ReS₂ compound is of considerable interest for various applications (e.g. sulfur-tolerant hydrogenation and hydrodesulfurization catalyst, a solar-cell material in electrochemical cells) due to its optical, electrical and mechanical properties. The ReS₂ framework with a trigonal unit cell (crystallizes in s distorted C6 structure) has the substructural motif consisting of Re₄ parallelogram units containing Re—Re metal bonds. IF-ReS₂ nanoparticles have been prepared by the direct sulfidization of ReO₂, formed from the decomposition of ReO_(3 [)14]. By adopting the MWCNT-templating approach, it has been also possible to prepare nanotubes of ReS_(2 [)15]. The crystal structure of MoS₂ (2H or 3R) is different than that of ReS₂ (C6). Therefore, it is not expected that the two different lattices would intermix and solid solutions of ReS2 and MoS₂ would be miscible. In addition contrary to other layered MS₂ compounds (MoS₂ and WS₂), ReS₂ contains in its bulk form metal-metal bonded clusters (Re₄) and metal atoms that are octahedrally rather than trigonal prismatically coordinated with sulfur. Consequently, 0.5% and 1% rhenium-doped (Re-doped) Mo(W)S₂ single crystals have been grown by the chemical vapor transport method with Br₂ as a transport agent. The Re doping was found to induce n-type conductivity of the Mo(W)S₂ crystal.

The following are some example of the preparation of nanostructures of present invention. Generally, characterization of all synthesized nanoparticles of the present invention was done in the following manner:

A vertical theta-theta diffractometer (TTRAX III, Rigaku, Japan) equipped with a rotating Cu anode operating at 50 kV and 200 mA was used for x-ray powder diffraction (XRD) studies. The measurements were carried out in the reflection Bragg-Brentano mode within the range of 10-70° of 2Θ-angles. XRD patterns were collected by a scintillation detector. The minute quantities of material available dictated a very slow data rate (0.05°/min) The peak positions and shapes of the Bragg reflections were determined by a self-consistent profile-fitting procedure using the Jade 8 software.

The following electron microscopes were used in this work: transmission electron microscope (Philips CM120 TEM) operating at 120 kV, equipped with EDS detector (EDAX-Phoenix Microanalyzer); HRTEM with field emission gun (FEI Technai F30-UT) operating at 300 kV, equipped with a parallel electron energy loss spectrometer [Gatan imaging filter-GIF (Gatan)]. For electron microscopy and analysis the collected powder was sonicated in ethanol and placed on a carbon-coated Cu grid (for TEM) or on lacy carbon-coated Cu grids (for HRTEM and EELS).

X-ray photoelectron spectroscopy (XPS) was carried out using a Kratos AXIS-HS spectrometer at a low power (75 W) of the monochromatized A1 (Kα) source. The samples for XPS analyses were prepared by depositing a few drops of the nanoparticles sonicated in ethanol, onto an atomically flat Au substrate (SPI supplies, thickness-150 nm) or onto Au polycrystalline films coating Si substrates.

A. Gas Phase Synthesis

Reference is made to FIGS. 2A-2B exemplifying an apparatus, generally designated 10, suitable to be used for the manufacture of such INT nanostructures. The apparatus 10 includes a vertical reaction chamber 12 associated with a separate therefrom and connectable thereto evaporation chamber 14. The reaction chamber 12 has first and second inlet units 16A and 16B arranged so as to enable to flow therethrough reacting materials towards one another in opposite directions. The inlet unit 16A serves for supplying a flow of vapors of two precursor compositions A-Y₁ and B-Y₂, where each of A and B is a metal or transition metal and Y₁/Y₂ is/are halogen(s) independently selected from chlorine, bromine or iodine, together with a reducing agent carrying forming gas. The inlet unit 16B located at the opposite edge of the vertical reactor serves for supplying a flow of a chalcogene carrying reacting gas.

As indicated above, metal or transition metal A may be one of the following: Mo, W, Re, Ti, Zr, Hf, Nb, Ta, Pt, Ru, Rh, In, Ga, and alloys like W_(x)Mo_(1-x), etc. and metal or transition metal B may be one of the following: Si, Nb, Ta, W, Mo, Sc, Y, La, Hf, Ir, Mn, Ru, Re, Os, V, Au, Rh, Pd, Cr, Co, Fe, Ni, where A and B are different and where B is to be doped to A-Y to obtain A_(1-x)-chalcogenide (i.e. x≦0.3, or lower as disclosed herein). In the case of In(Ga)S(Se) the dopant/alloying atoms can be In_(0.99)Ni_(0.01)S or Ga_(0.98)Mn_(0.02)Se.

In the present specific but not limiting example, the precursor compositions are MoCl₅ and NbCl₅; and the chalcogene carrying reacting gas is H₂S. The reducing agent carrying forming gas is H₂.

The reaction chamber 12 further includes a gas outlet 18, and a filter 20. The reaction chamber is associated with a heating unit 22 configured to form a two-stage furnace for the synthesis of e.g. Mo_(1-x)Nb_(x)S₂ nanoparticles: at the upper and lower parts of the chamber 12 different first and second temperature conditions T₁ and T₂ are provided.

Thus, vapors of MoCl₅ and NbCl₅ undergo a reduction reaction while interacting with H₂ gas during their flow towards a reaction zone where they meet H₂S reacting gas. The reduction reaction is thus followed by a reaction with H₂S resulting in the formation of Mo_(1-x)Nb_(x)S₂ nanoparticles.

The vapors of MoCl₅ and NbCl₅ are produced in the separate (auxiliary) evaporation chamber 14. It should be noted that generally separate chambers could be used for evaporating therein the two precursors, respectively, MoCl₅ and NbCl₅ in the present example. It should also be noted that H₂ gas can be supplied into the evaporation chamber thereby causing a reduction reaction to start while the precursor compositions are being evaporated in the evaporation chamber.

Example 1 Preparation of IF Mo_(1-x)Nb_(x)S₂ Nanoparticles

The synthesis of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles was carried out starting from the precursors MoCl₅ (Aldrich) and NbCl₅ (Alfa Aesar) in reaction with H₂S using the vertical reactor shown in FIG. 2A. Before each growth session, the vertical reactor being the reaction chamber 12 was preset to higher temperatures of about 600° C. and was purged continuously with N₂ in order to prevent traces of O₂ and water vapor that would otherwise interfere with the course of the reaction.

Precursors MoCl₅ (0.550 g; m.pt=194° C., b.pt=268° C.) and NbCl₅ (0.010 g m.pt=204.7° C., b.pt=254° C.) were first heated in the auxiliary furnace (evaporation chamber) 14 to a temperature of ˜250° C. (T₄). In order to avoid condensation of the vapors of the precursor, before it reaches the vertical reactor 12, a preset temperature of 220° C. (T₃) was maintained along the length of a tube (not shown) connecting the chambers 14 and 12 using a heating band. The vapors of the precursor were carried from below (i.e. through inlet 16A at the bottom edge of the reactor 12) into the hot zone (kept at a preset temperature, e.g. 900° C.) by flowing 50 cc of forming gas (I) (95% N₂ and 5% H₂). Forming gas was used to ensure complete reduction of the metal chloride precursors. Simultaneously, 5 cc H₂S (II) was introduced from above (i.e. through inlet 16B at the top edge of the reactor 12) mixed along with 50 cc of N₂ (III). The typical time period for each reaction was 30 min.

Two series of reactions (see Table 1 below) were carried out, wherein the temperature inside the reaction chamber 12 was maintained at (i) T₁=800° C. and T₂=850° C. (series-1) and (ii) T₁=850° C. and T₂=900° C. (series-2). At the end of the reaction, the product was collected (as a black colored powder) at the hot zone of the reactor 12 by means of the quartz wool filter 20, and was subsequently analyzed by various characterization techniques as detailed in Table 1 below.

TABLE 1 Two reactions for the preparation of IFs. Size of Physical obtained description Temp of IF- of vertical Temp. of Gas flow Mo_(1-x)Nb_(x)S₂ obtained IF- Reactor 12 Chamber 14 Rate nanoparticles nanoparticles Series-1 T₃ = 220° C. Forming gas ~50 nm Spherical IF T₁ = T₄ = 250° C. (I) = 50 cc; nanoparticles 800° C. (95% N₂; 5% H₂) T₂ = H₂S (II) = 5 cc 850° C. N₂ (III) = 50 cc Series-2 T₃ = 220° C. Forming gas ~40 nm IF T₁ = T₄ = 250° C. (I) = 50 cc; nanoparticles 850° C. (95% N₂; 5% H₂) More faceted T₂ = H₂S (II) = 5 cc 900° C. N₂ (III) = 50 cc

X-ray powder diffraction (XRD) studies were applied to the product using a vertical theta-theta diffractometer (TTRAX III, Rigaku, Japan) equipped with a rotating Cu anode operating at 50 kV and 240 mA. The measurements were carried out in the reflection Bragg-Brentano mode within the range of 10-70° of 2Θ-angles. XRD patterns were collected by a scintillation detector. The minute quantities of material available dictated a very slow data rate (0.05°/min). The peak positions and shapes of the Bragg reflections were determined by a self-consistent profile-fitting procedure using the Jade 8 software. XRD was carried out on both the IF-Mo_(1-x)Nb_(x)S₂ (from this work) and IF-MoS₂ nanoparticles (used as a reference).

FIG. 3 shows the XRD patterns obtained from for (a) IF-MoS₂ and (b) IF-Mo_(1-x)Nb_(x)S₂ nanoparticles (series-2) synthesized as described above prepared at T₁=850° C. and T₂=900° C. (series-2). FIG. 4 shows a zoom-in look of (002) peaks from FIG. 3. In both figures, standard diffraction patterns of 2H—MoS₂ (long line) and 2H—NbS₂ (short line) are shown for comparison.

A halo around 22° in FIG. 3 is due to the traces of amorphous quartz wool, which was used as a filter for collecting the synthesized nanoparticles. It is seen that the peaks of the first pattern (FIG. 3, curve a) matches IF-MoS₂ nanoparticles (used as a reference), while the second pattern corresponds to IF-Mo_(1-x)Nb_(x)S₂ nanoparticles (FIG. 3, curve b). The peaks in the IF-Mo_(1-x)Nb_(x)S₂ phase match well with that of IF-MoS₂. A detailed comparison between IF-MoS₂ and IF-MoNbS₂ (FIGS. 3 and 4) diffraction patterns shows some shifts of the (002) and (110) peaks. Comparing the (002) peaks at ˜14° for both IF-MoS₂ and IF-Mo_(1-x)Nb_(x)S₂ (FIGS. 3 and 4) it can be concluded that one of them (IF-MoS₂, FIG. 3, curve a and FIG. 4, curve a) has a symmetric shape while the other (IF-Mo_(1-x)Nb_(x)S₂ FIG. 3, curve b and FIG. 4, curve b) does not. The peak profile (˜14°, FIG. 4, curve b) is certainly not symmetric and probably consists of two peaks corresponding to c-axis spacings of 6.4 Å and 6.165 Å. The (110) peak at ˜58° of IF-Mo_(1-x)Nb_(x)S₂ (FIG. 3) has a very small shift (about 0.08°) to lower angles with respect to the IF-MoS₂ (110) peak. There are additional peaks at 2.88 Å (31°) and 1.66 Å)(55.3° which are best suited to the (100) and (110) reflections of the 2H—NbS₂ phase. Moreover, the peak profile around 31° exhibits an asymmetry (fast intensity increase and slow decrease with increasing angle) which is similar to the shape of the (100) reflection of IF-MoS₂ at 32.7° (FIG. 3, curve a). This latter peak is a typical example for the strongly asymmetric line shapes of the (h00) peaks of layered materials with relatively small number of stacked molecular layers. The absence of any other NbS₂ peak could be attributed to their relatively small concentration and their poor lattice order, which leads to low intensity of the NbS₂-related peaks in the noisy spectrum. All the other mixed (hkl) peaks would be expected to be suppressed completely if it is assumed that the NbS₂ structures form a turbostratically (misaligned layers) stacked system with randomness in translation and rotation of the layers. This may also be the reason for the broadening of the (002) peak (˜14°) in the case of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles.

The X-ray diffraction data presented above indicates that the as-synthesized IF-Mo_(1-x)Nb_(x)S₂ nanoparticles are comprised of two phases corresponding to those of (Nb doped) MoS₂ and NbS₂. The presence of a broad (002) peak (and the shift of this peak toward the position of (002) reflection of NbS₂) is indicative of the existence of fragments of a NbS₂ lattice incorporated turbostratically among the MoS₂ layers. Furthermore the presence of only (hk0) peaks of NbS₂ is indicative of the presence of the respective single layers. However, according to the Vegard's law, a small shift in the positions of the (110) peak in the case of IF-Mo_(1-x)Nb_(x)S₂ nanoparticles towards the position of (110) reflection of NbS₂ indicates substitution of a minor part (about 3%) of individual Mo atoms by individual Nb atoms into the MoS₂ structure (in addition to the NbS₂ nanosheets interspersed in the MoS₂ structure). It should be noted that the corresponding change of the lattice parameter is so small that an expected shift of another in-plane (100) peak at 32.7° will be comparable with the error of the measurement and consequently cannot be observed. Thus, in addition to the sheets/stripes of NbS₂ present turbostratically among the MoS₂ layers, there is also the replacement of Mo atoms individually by Nb atoms in the case of IF-Mo_(1-x)Nb_(x)S₂ nanoparticles.

Extensive TEM investigations were carried out on the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles. The following electron microscopes were used: transmission electron microscope (Philips CM120 TEM) operating at 120 kV, equipped with EDS detector (EDAX-Phoenix Microanalyzer); HRTEM with field emission gun (FEI Technai F30-UT) operating at 300 kV, equipped with a parallel electron energy loss spectrometer [Gatan imaging filter-GIF (Gatan)].

For electron microscopy and analysis the collected powder was sonicated in ethanol and placed on a carbon-coated Cu grid (for TEM) or on lacy carbon-coated Cu grids (for HRTEM and EELS). The energy windows for the elemental mapping by energy-filtered TEM (EFTEM) were chosen as follows (standard parameters of the software): Sulfur map was measured around the S L_(2,3) edge (167-187 eV); Niobium map was measured around the Nb L₃ edge (2370-2470 eV); Oxygen map was measured around the O K edge (532-562 eV).

Reference is made to FIGS. 5A to 5D showing the TEM and HRTEM images of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles: FIG. 5A shows a collection of IF-Mo_(1-x)Nb_(x)S₂ nanoparticles synthesized at T₁=800° C. and T₂=850° C. (series-1). As can be seen from the TEM image, the as-obtained IF nanoparticles are of uniform size distribution (diameter ˜50 nm). FIGS. 5B and 5C present the HRTEM images of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles prepared at T₁=850° C. and T₂=900° C. (series-2), respectively. The diameter of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles obtained by this reaction is about ˜40 nm. However, occasionally some very large IF nanoparticles of about 200 nm in diameter can also be observed. FIG. 5D shows the EDS spectra of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticle of FIG. 5C.

Niobium atoms are uniformly distributed in all the examined nanoparticles, irrespective of their size or shape. The presence of the characteristic and distinct Mo (K,L), S (K) and Nb (K,L) lines can be seen clearly. Comparing the IF nanoparticles of series-1 and series-2 cases, the series-2 IF nanoparticles are much more faceted and well crystallized. This is due to the higher synthesis temperature used in this case. The IF nanoparticle seen in FIG. 5B is reminiscent of a nanotube (nanotubes of undoped MoS₂ were obtained previously in the same temperature regime). The growth of the nanoparticles is very fast (<1 s). The supply of the precursor gasses is unlimited and the nanoparticles remain in the hot zone of the reactor until the reaction is turned down (30 min). These observations are indicative of that the nanoparticles cease to grow due to energetic consideration. Similar considerations were found to control the size of WS₂(MoS₂) nanotubes.

Referring to FIGS. 6A to 6C, there are shown the HRTEM images (FIG. 6A) and the corresponding EELS spectra (FIG. 6B) of an individual IF-Mo_(1-x)Nb_(x)S₂ nanoparticle prepared at T₁=850° C. and T₂=900° C. (series-2), and an exploded view (FIG. 6C) of a portion of the nanoparticle of FIG. 6A. As shown in FIG. 6A, the particle is well faceted with clear and sharp curvatures forming the closed-cage nanoparticle (diameter is ˜40 nm, number of layers ˜30). The EELS spectrum of FIG. 6B shows a good signal-to-noise ratio and very distinct Mo (L_(3,2)), S (K) and Nb (L_(3,2)) peaks. The Mo/Nb ratio was determined by the integration of the Nb-L_(3,2) edge relative to the Mo-L_(3,2) edge, after background subtraction. This gives an atomic ratio of Nb to Mo of about 0.30/1.00. The relative concentration of Nb in the Mo-compound in the product, as derived from EELS and from TEM-EDS analysis data, ranges between 15-25%. In order to ascertain whether Nb is present as an intercalant, between the MoS₂ walls, or in substitutional sites in the layers of the MoS₂ lattice, additional TEM-EDS and HRTEM-EELS analyses were performed. The results show that the Mo+Nb+S ratio remained nearly constant, independent of the IF nanoparticle diameter and position. The low loss region in the EELS spectra show two characteristic peaks: a plasmon peak at 23.3±0.1 eV, which is shifted to lower energies by 0.2 eV relative to the pure IF-MoS₂ sample (23.5±0.1 eV), and an additional feature at 8.2±0.2 eV. However, this change is too small to be used to differentiate between the two modes of Nb insertion, either in the layers of the host lattice (MoS₂) or as intercalating moiety in-between the MoS₂ layers. As shown in FIG. 6C, which is an enlarged view of a portion of the nanoparticle of FIG. 6A, a mismatch in the layers exists. FIG. 6D, which is the HRTEM of another IF nanoparticle, shows defects/dislocations in the layers.

The presence of layers' mismatch, defects and/or dislocations, in the case of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles, is another indication for the incorporation of Nb atoms in the lattice of MoS₂. The occurrence of such defects is not surprising considering the difference in coordination of the two metal atoms (trigonal biprism for the Mo and octahedral for the Nb atom). These kinds of defects were very rare in the case of the pure IF-MoS₂ nanoparticles [4a].

Reference is made to FIGS. 7A to 7C, where FIG. 7A shows the HRTEM image of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles prepared at T₁=850° C. and T₂=900° C. (series-2); FIG. 7B shows an expanded view of a portion of the particle of FIG. 7A; and FIG. 7C is a line profile of a framed region in FIG. 7B.

As revealed by the line profile of the framed area in FIG. 7B, the interlayer spacing is 6.40 Å. Although this spacing is observed in some cases (as described with respect to XRD study), this does not seem to be the rule for all the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles, since some of them exhibit the 6.20 Å spacing which is typical for the pure IF-MoS₂ nanoparticles. The observed expansion in the case of IF-Mo_(1-x)Nb_(x)S₂ nanoparticles might be too small to be attributed to intercalation of Nb in-between the layers. Furthermore, electron diffraction (ED) analysis of individual IF-Mo_(1-x)Nb_(x)S₂ nanoparticles does not support such intercalation. The diffraction pattern did not reveal any additional spots that may arise due to Nb intercalation.

The above results are also in good agreement with the XRD diffraction data. As mentioned above, NbS₂ may appear in two polytypes, hexagonal-2H (P63/mmc) and rhombohedral-3R (R3m). The XRD data of FIGS. 3 and 4 agree well with that of the 2H polytype. Thus, in this case, defects and dislocations in the layers, arising due to the incorporation of Nb in the layers, might be responsible for the observed increase in the layer spacing. Also, the fact that intercalation of IF nanoparticles with e.g. alkali metal atoms was found to be very non-uniform with internal layers of the IF nanoparticles unaffected at all, should be taken into account. The fact that Nb is distributed uniformly in the present IF-Mo_(1-x)Nb_(x)S₂ nanoparticles indicates that there is much less strain here as compared to the case of alkali metal intercalation in IF nanoparticles. Lattice substitution of Nb into the MoS₂ network is much less energetically demanding than intercalation, which leads to a large expansion in the interlayer distance.

Elemental mapping by energy-filtered TEM (EFTEM) analysis revealed the presence of Nb uniformly throughout the particles. Apart from the uniform Nb substitution into the MoS₂ lattice, there is also a very thin amorphous niobium oxide layer seen as an outer envelope on the IF nanoparticles. In this connection, reference is made to FIGS. 8A to 8D, showing images of elemental mapping by energy-filtered TEM (EFTEM) of IF-Mo_(1-x)Nb_(x)S₂ nanoparticle, with a surface oxide layer. FIG. 8A shows a zero-loss image; FIG. 8B shows the sulfur map measured around the S L_(2,3) edge (167-187 eV); FIG. 8C shows the niobium map measured around the Nb L₃ edge (2370-2470 eV); and FIG. 8D shows oxygen map measured around the O K edge (532-562 eV), which is indicative of a very thin NbO_(x) film covering the IF nanoparticle surface.

The stoichiometry of niobium oxide and the oxidation of niobium metal are of considerable interest, especially in the realm of superconductivity. Niobium oxide is known to exist in three principal forms: Nb₂O₅, NbO₂, and NbO, but several suboxides of the form NbO_(x)(x<1) are also known, and the structure of many of these have been reported. In the present case, however, since the top oxide film is amorphous, it was not possible to ascertain the exact phase of the niobium oxide layer sheathing the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles. A summary of all the data is presented in Table 2 below, comparing the 2H—MoS₂(2H—NbS₂), IF-MoS₂ (IF-NbS₂) and the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles.

TABLE 2 A summary of collected data Size of Additional information Polytypes particles Composition Type MoS₂ hexagonal-2H — — — (P63/mmc) c/2 = 6.15 A rhombohedral- 3R = 6.06 A NbS₂ hexagonal-2H — — — (P63/mmc) c/2 = 5.981 A rhombohedral- 3R(R3m) = 5.96 A IF-MoS₂ hexagonal-2H ~50 nm Mo:S Spherical/ (P63/mmc) 1:2 Faceted c/2 = 6.20 A nanoparticles IF-NbS₂ rhombohedral- 20-40 nm Nb:S Spherical 3R(R3m) (small 1:2 nanoparticles c/2 = 6.15 particles) to 5.9 A 60-80 nm depending (large on annealing particles) IF- Hexagonal-2H ~40-50 nm 15-25% Nb Spherical/ Mo_(1−x)Nb_(x)S₂ (P63/mmc) (90% small NbO_(%) Faceted c/2 - 6.40 A particles); surface nanoparticles ~200 nm layer (large particles 10%)

Thus, the TEM analysis reveals the presence of well faceted nanoparticles of diameter 40-50 nm. EDS and EELS measurements show the presence of Mo, Nb, S on the same individual nanoparticles, with Nb uniformly distributed over the entire nanoparticle. The concentration of Nb in each of the individual nanoparticles present is ascertained to be around ˜15-25 at % by TEM-EDS and HRTEM-EELS analysis. In some IF nanoparticles the interlayer spacing increases to 6.4 Å due to defects and/or dislocations within the layer arising as a result of Nb incorporation while in others the interlayer spacing remains at 6.2 Å (note that Nb is present uniformly in these IF nanoparticles as well). HRTEM-EELS chemical mapping reveals the presence of Nb distributed uniformly throughout the nanoparticles. The fact that the Nb is distributed randomly in the individual IF-Mo_(1-x)Nb_(x)S₂ nanoparticle and the absence of any local variations in the chemical composition excludes Nb intercalation as a potential major mechanism for altering the IF-MoS₂ lattice. Thus, Nb is incorporated within the MoS₂ layers. In addition from the EFTEM observations the presence of an oxide layer covering the nanoparticles is observed.

Reference is now made to Table 3 below and FIGS. 9 and 10A-10C, presenting the X-ray Photoelectron Spectroscopy (XPS) results. Table 3 summarizes the XPS data of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles (series-2) given as atomic concentration of the different elements.

TABLE 3 XPS data for IF-Mo_(1-x)Nb_(x)S₂ nanoparticles Element Atomic Concentration (%) Nb^(red) 0.37 Nb^(ox) 1.15 Mo 3.31 S 7.77 S^(ox) 0.78 O 3.15 C 30.6 Au 24.81

FIG. 9 illustrates the corresponding Nb 3d spectrum (i.e. line shape analysis of the Nb 3d signal with Gaussian-Lorenzian components. Here, reduced Nb (I and II) is believed to be within the nanoparticles. The oxidized Nb (III) appears on particle surfaces. More specifically: three doublets are shown in the figure; the two low-energy doublets correspond to a reduced Nb moiety, presumably within the dichalcogenide layers, while the high-energy doublet is associated with oxidized (external) Nb (as also revealed by energy filtered TEM). It should be noted that the last entry in Table 3, Au in an atomic concentration of 24.81, relates to Au from the substrate that is not part of the formed IF-Mo_(1-x)Nb_(x)S₂ nanoparticles. The high surface sensitivity of the XPS technique explains the large concentration of carbon which emanates from surface impurities.

The binding energies of Mo and S exhibit a marked difference when the Nb-substituted and the unsubstituted samples are compared: Mo(3d_(5/2)) at 228.9 and 229.3, respectively, and S(2p_(3/2)) at 161.7 and 162.1, respectively. This is clear evidence for incorporation of Nb into the Mo-based particles. The observed difference, which is practically identical for the Mo and S lines, 400±100 meV, is far beyond any possible charging effect (as will be described below with respect to CREM data). Also, it does not show up at the gold and carbon signals (and has a different magnitude for the oxygen). These findings indicate that this binding energy difference is associated with a Fermi level shift. Thus, the Fermi level of the Nb-substituted nanoparticles (IF-Mo_(1-x)Nb_(x)S₂) is shifted towards lower energies, making them more ‘p-type’.

A unique way to test the electrical properties of the nanoparticles is provided by the known technique of chemically resolved electrical measurements (CREM), which is a “top-contact-free” electrical characterization method. This technique allows for the determination of the electrical response of the different nanoparticles. By measuring the current flow to the ground and monitoring the energy shift of a given element in the surface layer during electron bombardment by a flood gun, the internal potential drop in the layer which contains this specific element can be determined.

FIGS. 10A-10C show electrically induced line shifts of Nb(3d) (FIG. 10A), Mo (3d_(5/2)) in IF-Mo_(1-x)Nb_(x)S₂ nanoparticles (FIG. 10B), and Mo (3d_(5/2)) in IF-MoS₂ nanoparticles (FIG. 10C) under a given electrical input signal (eFG filament current of 1.75 Å). The line shifts reflect the local potential changes at addresses associated with the inspected chemical element. In all panels, curve (I) corresponds to ‘eFG off’ conditions and curve (II) refers to ‘eFG on’. It should be noted that the oxidized Nb exhibits a large shift while the reduced Nb signal is practically not shifted at all. Comparison of FIGS. 10B and 10C demonstrate the effect of Nb substitution on the Mo line shift, indicating improved conductance in the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles. This experiment is conducted with the nanoparticles deposited on a gold substrate, and complementary CREM data (not shown) is recorded also for the gold, carbon and oxygen signals. All gold substrates exhibited small electron-beam induced shift of the XPS signal, signifying a relatively good Ohmic back contact.

The film of the IF-MoS₂ nanoparticles exhibits measurable line shifts upon electron irradiation with the eFG (Mo line in FIG. 10C). This line shift directly reflects a local potential change, due to the internal resistance of the nanoparticle (under the incident electron flux). The internal resistance of the nanoparticle is estimated to be on the order of a few hundred kilo Ohms In contrast, the Mo line of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles does not show any observable line shift (FIG. 10B), indicating that the nanoparticle resistance is low, such that only small, undetectable potential changes can evolve under the applied electrical signal (the input eFG current). Interestingly, the Nb line itself consists of two electrically different components (FIG. 10A). As mentioned above, the low binding energy components (203.70 eV and 204.80 eV) are attributed to atoms incorporated within the Mo-based particles, and they do not shift under the input flux of the electrons. The other component (at 208.40 eV), which is associated with external oxidized Nb species, does exhibit a strong line shift under the electron flux (FIG. 10A). This shift is similar in magnitude to that of the oxygen line (not shown). Based on complementary data (EFTEM described above), the oxidized Nb appears to comprise a subnanometer coating on the nanoparticle surface. This means that contact electrical measurements (e.g. with AFM, as will be described below) might be subjected to an oxide barrier, which is not the case for the non-contact CREM approach.

Finally, the CREM results are in good agreement with the observation of XPS-derived Fermi level shift upon Nb-substitution, which is manifested through the shift of the Mo and S lines to lower energies in the Nb substituted (alloyed) IF nanoparticles. The incorporation of Nb into the semiconducting IF-MoS₂ nanoparticles induces enhanced p-type behavior, where the Fermi level shifts down towards the valence band, and the electrical conductance increases accordingly.

Thus, the XPS analysis shows two low binding-energy doublets corresponding to reduced Nb moieties, presumably within the dichalcogenide layers, and a high-energy doublet associated with oxidized Nb, which is present on the surface (from complementary analysis, as also revealed by energy filtered TEM). One of these reduced species corresponds to the sheets of NbS₂, while the other one to alternate substitutional sites of individual Nb atoms at Mo atomic sites.

Reference is made to FIGS. 11A-11C, showing the results of conducting atomic force microscopy measurement (c-AFM) measurements carried out on an individual IF-Mo_(1-x)Nb_(x)S₂ nanoparticle (series-2), and also AFM measurements carried out on IF-MoS₂ nanoparticles for comparison. FIG. 11A shows the AFM image of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticle (series-2), FIG. 11B shows UV curves, and FIG. 11C shows corresponding dI/dV vs V plots. In FIGS. 11B and 11C, the corresponding plots of the IF-MoS₂ nanoparticle are shown for comparison (solid lines).

Whereas the IF-MoS₂ nanoparticle exhibits a noticeable bandgap region where no current flows, the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles exhibit only an inflection in the curve where current is reduced, but not to zero. Furthermore, the current rise is significantly sharper for the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles. Calculated dI/dV vs V traces of the curves in FIG. 10B, are shown in FIG. 10C. From these curves, the effective bandgap was measured. In both cases, the distribution is quite broad, representing both variations between the different particles, and experimental fluctuations. An average bandgap of ˜1.05 eV (±0.2 eV) was estimated for a canonical ensemble of the IF-MoS₂ nanoparticles. These results are indicative of that the IF-MoS₂ nanoparticles are semiconductors. The band gap of the bulk 2H—MoS₂ phase is 1.2 eV. The somewhat smaller gap of the nanoparticles, compared with the values of the bulk phase, may be attributed to the expansion of c/2 (vdW gap) in various regions of the nanoparticles, due to the strain involved in the folding of the structure. An alternative explanation for the reduced gap of the nanoparticles is that sub-bandgap states, which emanate from structural imperfections or edge dislocations, may serve as mediators for tunneling of charge and hence increase the current under bias. In contrast to the IF-MoS₂ nanoparticles, the IF-Mo_(1-x)Nb_(x)S₂ exhibits a metallic character. A minority of the particles measured (15%) showed an apparent gap of up to 0.6 eV which indicates the presence of an additional barrier in the tunneling gap. This is provided by the sheathed amorphous niobium oxide layer observed in the TEM measurements. It is possible that this layer is disrupted and penetrated by the tip in most measurements.

The experimental UV curves can be further analyzed to derive the effective resistance. The values were determined over a 0.3 V bias range starting directly after the current rises above the noise level. After accounting for the intrinsic point contact resistance of the tip as measured on the clean Au surfaces, the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles have resistance of 10 MOhm, as opposed to 60 MOhm for the undoped IF-MoS₂. Considering the existence of an oxide layer in some of the particles examined by TEM, part of this resistance is mediated by the quality of the contact.

Referring to FIGS. 12A to 12C, there are shown substitutional patterns of the Nb atoms within the IF-Mo_(1-x)Nb_(x)S₂ lattice, presenting three types of Nb incorporation within the lattice of MoS₂ based on the above described analysis of X-ray diffraction data, and TEM and XPS analysis. FIG. 12A represents the case where there are continuous spreads of both types of atoms within each layer alternating randomly (turbostatically). In FIG. 12B, Mo and Nb atoms are alternately incorporated into the lattice of MoS₂. It should be noted that if the concentration of the Nb atoms goes below 1% (or generally, does not exceed 1%), these atoms will generally prefer to be spread as individual atoms in the Mo-dominated lattice. In this case, the physical properties of the MoS₂ lattice, like the energy gap, are preserved. In this case the Nb atoms behave like a classical (hole) dopant making the lattice p-type. This situation provides the best conditions for the control of the electrical properties of the lattice, from intrinsic (undoped) to the case where as much as 10²⁰ cm⁻³ Nb atoms are substituted into the Mo sites, making it a heavily doped hole (p-type) conductor. FIG. 12C shows the unlikely case of intercalation of atoms in the van der Waals gap between the layers. The Nb incorporation could be a combination of the first two types.

The two kinds of Nb species could have a different effect on the electronic properties of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles. Individual Nb atoms in substitutional sites could play the role of a dopant, leading to a downwards shift of the Fermi level closer to the valence band and increased conductivity. Patches of NbS₂ sheets interspersed in the MoS₂ lattice impose a metallic character on the nanoparticle. The apparent ‘soft gap’, manifested by the reduced (but not totally suppressed) current around zero bias, as measured by conductive AFM, could be also influenced by the size of the nanoparticles, with the larger nanoparticles exhibiting larger gap [11]. Hence, the Nb incorporation into the 2H—MoS₂ lattice of the IF reduces the resistivity of the nanoparticle substantially.

Thus, the present invention provides novel IF-nanostructures, generally of the formula A_(1-x)-B_(x)-chalcogenide, where A is a metal or transition metal doped by another metal or transition metal B different from A, and x≦0.3. In the above described not-limiting example, IF-Mo_(1-x)Nb_(x)S₂ nanoparticles have been prepared starting from the respective chloride vapor precursors in addition with H₂S. The IF-Mo_(1-x)Nb_(x)S₂ nanoparticles have been extensively characterized by XRD, TEM-EDS, HRTEM-EELS and XPS. From the detailed investigation of the electrical properties by AFM analysis, the substitution of Nb for Mo involves a semiconductor to metallic type transformation (IF-MoS₂ are known to be semiconductors). This study is an example of heteroatom substitution in the case of IF-nanoparticles opening up a wide range of possibilities including varying the electronic behavior of the IF-nanoparticles, in superconductivity and in spintronics.

Example 2 Synthesis of Mo_(1-x)Re_(x)S₂ IF-Nanoparticles

The synthesis was carried out in a method similar to the above-described synthesis of the IF-Mo_(1-x)Nb_(x)S₂ nanoparticles. The precursors in this case are MoCl₅ (m.pt=194° C.; b.pt=268° C.) and ReCl₅ (m.pt=220° C.).

FIGS. 13A and 13B show together an experimental setup used in the present example. Reference numbers are the same as those used in FIG. 2. Initially, the precursors were heated (temperature T₃) in a separate auxiliary furnace 14 (FIG. 13B) and the respective vapors were carried while heated at a temperature T₂ into the main reactor 12 (FIG. 13A). The latter being a horizontal reactor, although one may carry the reaction in a vertical reactor similar to that described previously in the case of the synthesis of IF-Mo_(1-x)Nb_(x)S₂ nanoparticles, with reference of FIG. 2. As for the horizontal reactor, which was previously used for the synthesis of IF-TiS₂ nanoparticles, it now has been modified for the synthesis of the IF-Mo_(1-x)Re_(x)S₂ nanoparticles and nanotubes. The MoCl₅ and ReCl₅ vapors were initially formed from the respective solid material at the auxiliary/precursor furnace 14 (FIG. 13B). The temperature of the precursor source was kept usually between 220° C. and 250° C. which is close to the boiling point of the chlorides. The formed vapors were introduced into the main reactor 12 (FIG. 13A) through inlet 16A by a carrier gas flow (N₂/H₂). The H₂S gas diluted with N₂ were provided from opposite directions through inlet 16B of the reactor 12 respectively. This enables the reaction to occur in the central hot region of the reactor (associated with a respective furnace), while the product is swept by the flow and collected onto the filter (as described above with reference to FIG. 2). Excess gas leaves the reactor through the opening 18.

The preheating temperature was found to be an important factor determining the amount of precursor supplied to the reaction. The flow-rate of nitrogen through the bottle (10-100 cc/min) affects the stream of the tungsten chloride precursors as well. A small overpressure (1.1 bar) was maintained by using a toxic gas trap filled with NaOH (5%) solution in the gas outlet of the reactor. The temperature of the reaction chamber, where the two gases (MoCl₅ and H₂S) mix and react, was varied in the range of 800-900° C. The resulting Mo_(1-x) Re_(x)S₂ powder was collected using a filter. Unlike the case of TiS₂ synthesized previously, by the use of the modified horizontal reactor and by collecting the product in the filter, the main portion of the product could be collected without losses, and was not swept away by the carrier gas to the trap. The flow-rate of H₂S (5-10 cc/min) was controlled by means of a TYLAN model FC260 mass flow controller. The H₂S was diluted by mixing this gas with a stream of N₂ gas (10-200 cc/min in this reaction) using another flow-controller.

Table 4 below shows the parameters and conditions of the reactions carried out for the synthesis of the IF-Mo_(1-x)Re_(x)S₂ nanoparticles.

TABLE 4 parameters and conditions of reactions carried out for the synthesis of IF-Mo_(1-x)Re_(x)S₂ nanoparticles. Temp of the Temp of the Size of the horizontal auxiliary IF-Mo_(1-x)Re_(x)S₂ reactor furnace Gas flow rates nanoparticles Series-1 T₂ = 220° C. Forming gas ~30-80 nm  T₁ = 800° C. T₃ = 250° C. (I) = 50 cc (95% N₂ and 5% H₂) H₂ S (II) = 5 cc N₂ (III) = 50 cc Series-2 T₂ = 220° C. Forming gas  50-80 nm T₁ = 850° C. T₃ = 250° C. (I) = 50 cc (95% N₂ and 5% H₂) H₂S (II) = 5 cc N₂ (III) = 50 cc Series-3 T₂ = 220° C. Forming gas 50-100 nm T₁ = 900° C. T₃ = 250° C. (I) = 50 cc (95% N₂ and IF 5% H₂) nanoparticles + H₂S (II) = 5 cc nanotubes N₂ (III) = 50 cc

IF-Mo_(1-x)Re_(x)S₂ nanoparticles synthesized at 800° C. (Table 4, Series-1) resulted in both spherical and well-faceted nanoparticles of approximately 30 to 80 nm in diameter and interlayer spacing of approximately 0.62 nm TEM-energy dispersive X-ray spectroscopy (EDS) and high resolution TEM-electron energy loss spectroscopy (HRTEM-EELS) analysis reveal the presence of Re in the nanoparticles.

The HRTEM images of the IF-Mo_(1-x)Re_(x)S₂ nanoparticles synthesized at 850° C. (Table 4, Series-2) are shown in FIGS. 14A and 14B. The diameter of the IF-Mo_(1-x)Re_(x)S₂ nanoparticles is in the range of 50-80 nm in diameter. The EDS spectra of the IF-Mo_(1-x)Re_(x)S₂ nanoparticle are shown in FIG. 14B and FIG. 14C. Rhenium atoms seem to be uniformly distributed in all the examined nanoparticles, irrespective of their size or shape. The presence of the characteristic and distinct Mo (K,L), S (K) and Re (M,L) lines can be clearly seen. From the TEM-EDS and the HRTEM-EELS analysis the metal to sulphur ratio is determined to be 1:2. The stoichiometry of the IF-Mo_(1-x)Re_(x)S₂ nanoparticles is as follows: Mo:Re:S—0.97(±0.01):0.03(±0.01):2. Additional TEM-EDS and HRTEM-EELS analyses show that the Mo+Re+S ratio remained nearly constant, independent of the IF nanoparticle diameter and position.

FIG. 15A shows a TEM image of a collection of the IF-Mo_(1-x)Re_(x)S₂ nanoparticles synthesized at 900° C. (Table 4, Series-3). FIG. 15B shows the HRTEM image of a single nanoparticle. The diameter of the nanoparticles is between 50-100 nm at this temperature of synthesis. The presence of Re is ascertained from the EDS analysis carried out on individual as well as a collection of IF-Mo_(1-x)Re_(x)S₂ nanoparticles (FIG. 15D). The presence of the characteristic and distinct Mo (K,L), S (K) and Re (M,L) lines can be clearly seen from the EDS spectra. Apart from obtaining IF-Mo_(1-x)Re_(x)S₂ nanoparticles, nanotubes of Re-doped MoS₂ were also obtained at 900° C. (Table 4, Series-3). The nanotubes come in small amounts (˜5%). FIG. 15C shows the HRTEM image of one such nanotube. The length of the nanotubes thus obtained is about half a micron, whereas the diameter is about ˜40 nm (˜25 layers). The interlayer spacing of the nanotube shown in FIG. 15C is ˜0.62 nm, which is very similar to that of pure IF-MoS₂. The EELS spectra of nanoparticles and nanotubes showed the characteristic Mo (L_(3,2)), S (K) and Re (M_(4,5)) signals and the amount of Re in the particles was about 1-2 atomic percent. Higher temperatures of synthesis were found to be suitable for the production of the nanotubes. This observation was similar to the synthesis of pure MoS₂ and WS₂ nanotubes wherein higher temperatures of synthesis (˜900° C.) favoured their formation [4].

FIG. 16 shows the XRD pattern of the IF-Mo_(1-x)Re_(x)S₂ nanoparticles synthesized at 850° C. (Table 4, series-2,) and at 900° C. (Table 4 series-3). The standard diffraction patterns of 2H—MoS₂, 2H—ReS₂ are also shown in the figure for comparison. It can be seen that all the peaks of the sample match well with those of 2H—MoS₂. The (002) peak in the XRD pattern was characterized by a shift to a lower angle as compared to the (002) peak in hexagonal 2H—MoS₂ crystals indicating a small lattice expansion in the case of the IF nanoparticles and nanotubes [2-4]. This expansion has been attributed to the introduction of strain owing to curvature of the layers [1-4]. Comparison of the full width at half maximum of the (002) peaks (FIG. 16) confirm the TEM data that the size of the nanoparticles obtained at 900° C. is larger than those nanoparticles obtained at 850° C. Any secondary phases of ReS₂ have been ruled out from the XRD pattern. However the presence of a small proportion of ReO₃ and MoO₂ is present as can be seen in the figure. This is in accordance with the TEM observations wherein it is seen that the core of some of the nanoparticles reveal the presence of an oxide ReO₂ and ReO₃ that may undergo sulfidization). Table 5 summarizes the XPS derived atomic concentrations of the IF-Mo_(1-x)Re_(x)S₂ nanoparticles (Table 4, series-3).

TABLE 5 XPS-derived compositions of the IF-Mo_(1-x)Re_(x)S₂ nanoparticles (Series 3) given in atomic percentages. Element Atomic Concentration [%] Re 0.02 Mo 2.98 S 5.78 O 37.55 C 28.50 Si 21.98

The characteristic Re (4f) signal is clearly seen along with that of Mo (3d_(5/2)) and S (2p_(3/2)) in the spectra, however its quantity (˜1%), is of a relatively large uncertainty, due to the neighbouring Mo (3p) signal. The values in Table 5 is an average over a number of experiments (10 experiments) and is in accordance with the atomic percentage of Re obtained with HRTEM-EELS.

The binding energies of Mo and S exhibit a marked difference when the Re-substituted and the unsubstituted IF samples are compared. This is a clear evidence for the incorporation of Re into the Mo-based particles. The observed difference, is practically identical for the Mo and S lines, Δ=200±100 meV, and is verified to be beyond any possible charging effect. In these experiments, the charging conditions of the sample were changed systematically by varying the flux of the electron flood gun. Additionally, reference lines like that of the gold substrate and the carbon contamination do not show the relative shifts in the binding energy. It is therefore concluded that the Δ shift arises from the Re incorporation into the lattice which raises the E_(F) upwards, thus making the nanoparticles more n-type.

Turning to FIG. 17, the figure demonstrates schematically a possible MoS₂ nanoparticle having a Re atom incorporated therein.

Example 3 Synthesis of W_(1-x)Re_(x)S₂ IF-Nanoparticles

The synthesis was carried out in a method and an apparatus similar to the above-described synthesis of the IF-Mo_(1-x)Re_(x)S₂ nanoparticles. The precursors in this case are WCl₄ (m.pt=300° C.) and ReCl₅ (m.pt=220° C.).

The temperature of the precursor source was kept usually between 275° C. and 325° C. which is close to the boiling point of the chlorides. The preheating temperature was found to be an important factor determining the amount of precursor supplied to the reaction. Table 6 shows details of the reactions carried out for the synthesis of the IF-W_(1-x)Re_(x)S₂ nanoparticles.

TABLE 6 details of reactions carried out for the synthesis of IF-W_(1-x)Re_(x)S₂ nanoparticles. Temp of the Temp of the Size of the horizontal auxiliary IF-W_(1-x)Re_(x)S₂ reactor furnace Gas flow rates nanoparticles Series-1 T₂ = 300° C. Forming gas ~100 nm T₁ = 850° C. T₃ = 325° C. (I) = 100 cc (95% N₂ and 5% H₂) H₂S (II) = 10 cc N₂ (III) = 50 cc Series-2 T₂ = 300° C. Forming gas 50-75 nm T₁ = 900° C. T₃ = 325° C. (I) = 100 cc (95% N₂ and 5% H₂) H₂S (II) = 5 cc N₂ (III) = 50 cc

Shown in FIGS. 18A, 18B are the HRTEM images of IF-W_(1-x)Re_(x)S₂ nanoparticles synthesized at 900° C. (Table 6, Series-2). The diameter of the IF-W_(1-x)Re_(x)S₂ nanoparticles is found to be in between 50-75 nm in this set of experimental conditions. The nanoparticle in FIG. 18A is well-elongated whereas the nanoparticle in FIG. 18B is clearly faceted—the differences arising from different temperatures (above 850° C.). The interlayer spacing of the particle is found to be 0.62 nm as shown by the line profile in FIG. 18D (inset). The similarity to the value of the interlayer spacing of pure IF-WS₂ suggests that here again the Re is present in lower percentages. The presence of Re is confirmed by TEM-EDS analysis. Shown as an inset in FIG. 18C is the EDS spectrum revealing the characteristic W(L,M), Re(L,M) and S(K) lines. The composition as ascertained from TEM-EDS analysis is found to be as follows: W:Re:S—0.97(±0.01):0.03(±0.01):2.

In the present case both XRD and HRTEM indicate that Re is present as a dopant in the lattice (for both MoS₂ and WS₂) and not as an intercalant since the presence of Re as an intercalant would result in an additional lattice expansion in the spacing of the (002) layers.

The following are additional examples of the reactors set-ups and experimental methods for the synthesis of MoS₂, WS₂ doped with Re and Nb.

B. Solid Phase Synthesis

FIG. 19A shows the quartz reactor used for the synthesis of the Mo_(1-x)Re_(x)S₂ (x<0.02-0.7 at %, i.e. 200-7000 ppm), together with the temperature profile in the furnace along the reaction path. Here, MoO₃ powder and H₂S gas were used as precursors in a reducing atmosphere. The vertical reaction chamber 12 (associated with a separate evaporation chamber which is not specifically shown here) was configured and operable for the manufacture of IF nanostructures of the present invention. The same reference numbers are used for identifying components that are common in all the examples of the reactor set-ups.

In this example, simultaneous evaporation of a pre-prepared Re_(x)Mo_(1-x)O₃ (x<0.01) oxide takes place at 770° C. The Re_(x)Mo_(1-x)O₃ powder was prepared in a specially designed auxiliary reactor, depicted in FIG. 19B, in the following manner: weighted ReO₃ and MoO₃ powders are ground carefully in air. Afterwards, ethanol is added to the mixture in the volume ratio 2:1 ethanol/powder ratio and is sonicated by horn (Ultrasonic Power Model 1000 L, 100 W) for 5-10 min. After drying in air, the powder is ground again and put in crucibles and is inserted into the auxiliary horizontal reactor. The temperature is gradually raised from 300 C to 750° C. (1 deg min⁻¹) under nitrogen atmosphere and maintained for 45 min. At this point the boat with crucibles is moved out of the oven and cools down to room temperature under nitrogen flow. The crucibles are then inserted into the main reactor (FIG. 19A) for the sulfidization reaction.

In the main vertical reactor, the reaction chamber 12 has first and second inlet units 16A and 16B for flowing therethrough reacting materials H₂S+H₂/N₂ and N₂ respectively; an outlet unit 18 for gas exit; and a filter 20. The reactor 12 is made of three concentric quartz glass tubes 12A, 12B and 12C which are placed in a three zone oven. A series of crucibles containing the pre-prepared Re_(x)Mo_(1-x)O₃ powders, are placed in the middle concentric tube 12A.

The oxide vapors react with hydrogen gas above the crucibles at 800° C. This gas phase reaction leads to a partial reduction of the vapor and its condensation into MoO_(3-y) nanoparticles. The resulting rhenium doped molybdenum oxide nanoparticles react with H₂/H₂S gas at 810-820° C. converting the core into MoO₂ and the surface is engulfed by a mono- or bilayer of MoS₂ which coats the entire oxide nanoparticle surface. The sulfidization of the NP surface prevents the ripening of the nanocrystals into bulk 2H—MoS₂. Once landing on the filter 20, the MoO₂ to sulfide conversion reaction continues via a slow in-diffusion of sulfur and out-diffusion of oxygen. To complete this oxide to sulfide conversion a long (25-35 hr) annealing at 870° C. in the presence of H₂S and forming gas was performed. At the end of this process a powder of Re-doped MoS₂ nanoparticles (Re:MoS₂NP) was obtained.

The Re concentration being less than 1 at % means that many techniques, like EDS, XRD, etc., can resolve the rhenium signal, at least in the highest concentration, but are unable to determine its concentration quantitatively. Therefore, the Re concentration in the MoS₂NP was measured by time of flight secondary ion mass spectrometry (TOF-SIMS), presented in FIG. 20 and inductive coupled plasma mass spectrometry (ICP-MS) shown in FIG. 21. Indeed the Re level in the nanocrystallites was found to increase with the formal ReO₃ content of the pre-prepared oxide powder. The Re/Mo ratio varied to some extent from one measurement to the other (3 measurements for each Re concentration in the two methods). These results suggested that, in spite of the co-evaporation of the two oxides, the present synthetic procedure results in some nonuniformity in the doping level. Both TOF-SIMS and ICP-MS techniques determine the Re level in the MoS₂ powder as an ensemble average. Therefore, it was impossible to determine if the non uniformity in the Re concentration occurs within the individual nanoparticles, or the Re level varies from one zone of the collected powder to the other (see description below). Furthermore, the Re/Mo ratio values were lower than the formal concentration and there was some, though not large, discrepancy between the two methods. This difference could be possibly attributed to the different methods used for sample preparation, or to small variations in the batch to batch due the synthetic procedures. Roughly speaking therefore, the average Re concentration in the MoS₂NP was found to be about 1/3 to 1/2 of the formal (weighted) concentration in the oxide precursor. Given the fact that the average sized nanoparticles consists of about two hundred thousand Mo atoms, this means that each NP contains about 100 Re atoms (in the 0.12 at % sample).

Typical high resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (HRTEM) micrographs of the NP are shown in FIGS. 22A and 22B. No impurity, like oxides, or platelets (2H) of MoS₂ could be found in the product powder. The growth mechanism of the Re:MoS₂NP can be described as self-arresting, i.e. no further growth or ripening of the nanoparticles with reaction-time could be observed. The line profile and the Fourier analyses (FFT) of FIG. 22B show an interlayer spacing of 0.624 and 0.627 nm, respectively, coinciding with that of undoped MoS₂ fullerene-like NP. Furthermore, the layers seem to be evenly folded and closed with very few defects and cusps, demonstrating the nanoparticles to be quite perfectly crystalline. Powder X-ray diffraction presented in FIG. 23 and Raman analyses did not reveal particular differences with respect to the undoped material. Careful X-ray photoelectron spectroscopy (XPS) analysis showed that the sample with 0.71 at % formal Re concentration contained ˜0.46 at % rhenium. The oxidation state of the rhenium was +IV. Furthermore, the Re was found to be bound to sulfur atoms, i.e. it makes part of the MoS₂ nanoparticle lattice. Energy dispersive X-ray (EDS/SEM and TEM) and electron energy loss (EELS/TEM) spectroscopies (Re M4,5 edge at 1883 eV) revealed that the Re distribution in all the measured samples was inhomogeneously distributed among different groups of nanoparticles with concentrations of below 1 at %.

A few additional facts in connection with the doped nanoparticles are noteworthy. First the doped nanoparticles powder looks fluffier than the undoped one, suggesting lower degree of agglomeration of the former. Furthermore, it appears that the doped nanoparticles form a more homogeneous dispersion in various solvents. In particular, sedimentation experiments in oils were carried out, as follows: PAO-4 paraffin oil with a viscosity of 27 cP at 20° C. was used as a base oil. 1 wt % of the MoS₂ nanoparticles with an average size close to 80 nm was added to the paraffin oil. A magnetic mixer and magnetic stir bars were used in this work. The mixing time was 1 h. The sedimentation analysis was performed during 24 h. A weight of the nanoparticles_sediments was measured with an accuracy of 0.1 mg. No surfactants were applied for dispersion of the nanoparticles in the lubricant. These experiments results, presented in FIGS. 24A-24B demonstrate that, in contrast to the undoped nanoparticles, which exhibit significant sedimentation after 24 hr leaving translucent oil above, part of the doped nanoparticles remain in the suspension, possibly indefinitely. These results suggest that the doping of the NP is probably not uniform. Alternatively, some agglomeration of the doped nanoparticles leads to their partial precipitation. Notwithstanding the fact that no surfactant was added to the suspension however, the doped nanoparticles form a stable oil suspension, apparently for indefinite time. In a separate experiment, the undoped and 0.12 at % doped MoS₂ nanoparticles were spread on a Si wafer coated with a thin gold film. Here, a few milligrams of the nanoparticles were held in a container which was inserted to a vacuum chamber. When a valve was opened and air was allowed to rush through the container into the vacuum, a cloud of the nanoparticles was formed which slowly landed on the underlying substrate. FIGS. 25A-25B shows typical SEM pictures of the two samples. The Re-doped sample (FIG. 25A) showed a much better spreading of the nanoparticles and smaller cluster sizes (FIG. 25B) on the gold substrate. When 2 mg of the nanoparticles were immersed in 20 ml of water the pH of the suspension dropped from 6.8 to 6.3 (undoped MoS₂ nanoparticles) and 6.5 for the Re (0.12 at %):MoS₂NP. This indicates that the doped NP are negatively charge on the surface. This negative charge is partially neutralized by the adsorbed H⁺ ions leaving OFF ions in the solution.

Measuring the transport properties of pressed pellets of these nanoparticles proved to be quite demanding. The pellets were found to be exceedingly brittle and easily prone to mechanical failure, possibly due to the loose contact between the nanoparticles. This brittleness could be attributed to their mutual repulsion due to the negative surface charge. This observation was particularly true for the doped nanoparticles. While the inertness of the surface prevented a firm contact between the nanoparticles in the pressed pellet, it was found to be a boon for the tribological behavior. Furthermore, metallic electrical contacts could not be easily established on the pellets. FIG. 26A shows the electrical resistivity vs. temperature of a number of Re:MoS₂ nanoparticle pellets with different Re content. Also included is resistivity vs. temperature data for bulk (2H—MoS₂) powder. Generically, the resistivity increases with decreasing temperatures for all the samples reflecting a semiconductor behavior. Nonetheless, the specific resistivity goes down with the Re doping level. The activation energy for ionization of the dopant atoms at 300 K was calculated using a fluctuating barrier heights model. The results of this analysis (see FIG. 26B) indicate that the activation energy went from approximately 0.35 V for the undoped to less than 0.1 eV for the Re doped nanoparticles. The low activation barrier for the transport of charge carriers in the Re:MoS₂ nanoparticles (<0.1 eV) is characteristic to heavily doped semiconductors. These results also agree quite well with the theoretical calculations for Re doped MoS₂ nanotubes which predict a donor level 150 meV below the conduction band edge.

Tribological measurements using flat pin (0.09 cm²) on disk set-up were carried out using low viscosity poly-alpha-olefin (PAO) type 4 (18 mPa×s (cP) at 40° C.) fluid, according to the following method. The tribological measurements were carried out using pin (flat head-0.09 cm²) on disk set-up. In order to achieve “mixed” lubrication conditions, low viscosity (18 mPa×s at 40° C.) PAO-4 oil and slow velocity (0.24 m/s) were used. The disc was made of a hard stainless steel AISI 4330, while for the pin a soft steel (AISI 1020) was chosen. A drop of oil was added to the contact area at a rate of 1 drop every 30 s. In the run-in period, the load was gradually increased to valued (500-600 N) until a stable friction coefficient of (0.08-0.1) was obtained in the pure oil. The gap between the metal surfaces was calculated using the equation: ^(S3)λ=t/√2R_(q). Here R_(q) is the surface roughness of the soft metal (pin) and λ is a parameter characteristic of different lubrication regimes, and t is the gap between the two metal surfaces. It has been determined ^(S3) that for “mixed” lubrication this parameter varies between 0.05 and 3. Below λ=0.05 “boundary” lubrication is prevalent, i.e. the gap between the reciprocating metal surfaces disappears. Using the value of 0.05 and the average measured surface roughness in the case of PAO-4 (1.8 μm), t was calculated to be 127 nm.

The results of the tribological experiments are shown in FIG. 27A. The applied loads were in the range of 500-600 N leading to pressure in the range of 55-66 MPa. This pressure together with the relatively low velocity (0.24 m/s) and the low fluid viscosity provided “mixed” lubrication conditions, i.e. the friction coefficient was 0.08-0.1 at the end of the run-in period in the pure oil. The calculated gap of 127 nm coincides with the size of the NP, allowing them to free access to the contact area. Long (3-7 days) and careful run-in periods in the pure oil preceded the reported measurements. During the run-in period the load was gradually increased to the testing conditions (500-600 N) until stable values of the friction coefficient (0.08-0.1); temperature (40±3° C.), wear rate (8.5×10⁻⁷ mm³/Nm) and relatively large average surface roughness parameter of 1.8 μm (see FIG. 27B) were reached. At this point, the NP-oil lubricant was added to the interface drop-wise. The temperature in the contact area was gradually reduced to 30±3° C. FIG. 27A summarizes some of the typical tribological measurements of the pure oil; pure oil with 1 wt %; MoS₂NP, and the oil with 1 wt % of Re (0.12 at %):MoS₂NP. The pure PAO-4 oil showed a friction coefficient of close to 0.09. The friction coefficient of PAO-4 with 1% MoS₂NP was found to be between 0.04 and 0.05. Remarkably, the friction coefficient of the oil with 1 wt % Re (0.12 at %):MoS₂NP gradually decreased to 0.014-0.015 and the surface roughness parameter went all the way down from 1.8 to 0.1 μm (see FIG. 27B). These measurements were repeated a number of times with high reproducibility. The table in the insert of FIG. 27A shows that the Re-doped MoS₂ nanoparticles also exhibit, by-far, the lowest wear rate (4.7×10⁻⁷ mm³/Nm). Interestingly, replacing the oil+Re doped nanoparticles with a pure oil suspension did not lead to any noticeable change in the low friction and wear rate of the metal pairs for at least additional 24 hr. The low friction; smooth surface and the lengthy period of friction in pure oil after the friction test in the oil+Re doped NP indicate the formation and preservation of an easy shear film at the contact surface.

It is therefore believed that the gradual decrease in friction and wear could be attributed to the build-up of a film with appreciable conductivity on both sides of the matting metal contact. These results indicate that both the friction coefficient and the wear rate are adversely affected by tribocharging of the interface, which problem is at least partially relieved by doping the nanoparticles. Furthermore, the agglomeration of the undoped nanoparticles, which hinder their facile access to the interface, is largely alleviated in the case of the doped nanoparticles.

It is also believed that the free-carriers distribution within the individual nanoparticle is not uniform. In fact, due to a Columbic repulsion, the free carriers are likely to spend most of their time on the outermost layers of the fullerene-like nanoparticles and nanotubes. Most likely, the free carriers are trapped at surface defects inducing negative charge. Such defects occur when the surface-monolayer folds in sharp angles and the sulfur atoms are replaced by oxygen or OH moieties in the kinks. Thus, the free electrons are trapped at these states and induce negative charge on the nanoparticles' surface. The negative surface charge can be balanced by either an inner space-charge layer (band bending); by external double layer or chemically adsorbed positive ions, and possibly jointly by all of these factors. Nonetheless, these surface charges lead to mutual repulsion at close proximity and prevent agglomeration and subsequent sedimentation of the nanoparticles. Consequently, the nanoparticles can access the tribological interface more easily, providing both easier shearing and a slower wear rate. Furthermore, once the fullerene-like nanoparticles start to exfoliate they form protecting films which undergoes oxidation to MoO₃. Re doping of MoO₃ is known to result in a conductive film, which is also advantageous for their tribological action in comparison to the undoped oxide film. Careful doping of the fullerene-like NP and nanotubes could lead to other advances for example in the electronic and optical properties of such NP for sensor applications, etc.

Example 4

Referring to FIG. 28, there is shown a vertical reaction chamber 12 (associated with a separate evaporation chamber which is not specifically shown here) configured and operable for the manufacture of IF nanostructures of the present invention. The same reference numbers are used for identifying components that are common in all the examples of the reactor set-ups. In the present example, the reactor 12 is used for the growth of Re (Nb) do-poped IF-MoS₂ nanoparticles from MoO₃ and ReO₃ (Nb₂O₅) powder and H₂S gas. The reaction chamber 12 has first and second inlet units 16A and 16B for flowing therethrough reacting materials H₂S and H₂/N₂; an outlet unit 18 for gas exit; and a filter 20. The reactor 12 is made of three concentric quartz glass tubes 12A, 12B and 12C which are placed in a three zone oven 22. A series of 10 crucibles, generally at 24, are placed in the middle concentric tube 12B. Each crucible 24 has smaller and larger concentric parts. The powder of metal or transition metal B oxide precursor (0.5 g of MoO₃ in the present example) is placed in the outer crucible, and the metal or transition metal A oxide precursor (<1 wt % of ReO₃ in the present example) is placed in the inner part of the crucibles. The temperature profile of the reaction is shown also in the figure.

After 3 hr the product is retracted from the main reactor 12, and the annealing step is continued for another 20-30 hr at 860-870° C. in another (auxiliary) reactor 120 shown in FIG. 29. The reactor 120 has inlet unit 16, outlet 18, filter 20, and is formed by two concentric tubes 120A and 120B mounted in a three zone oven 22.

FIG. 30A shows a scanning electron microscopy (SEM) of a typical Re-doped IF-MoS₂ nanoparticle synthesized in this fashion. The IF-MoS₂ nanoparticles appear to be squashed. Careful examination of the product shows that the entire product consists of such nanoparticles with no bulk (2H platelets) analogues of MoS₂. FIG. 30B shows a transmission electron microscopy (TEM). This and other TEM images show that in terms of crystalline perfection, one sees no difference between those nanoparticles and the ones synthesized without Re.

Example 5

In this example the main reactor 12 is generally similar to that of FIG. 28, but instead of using the two oxide MoO₃ and ReO₃ precursors separately, a new auxiliary reaction chamber 130 shown in FIG. 31 is used to prepare a solid solution of Re_(x)Mo_(1-x)S₂ from Re_(x)Mo_(1-x)O₃ powder. To this end, the Mo_(x)Re_(1-x)O₃ powder is prepared in the following manner: Two weighted powders are grinded carefully in air. Afterwards, ethanol is added to the mixture in the ratio 2:1 ethanol/powder ratio and left in an ultrasonic bath for 5 min under high intensity irradiation. After drying in air, the powder is grinded and again put in crucibles and insert to the horizontal reactor 130 of FIG. 31. The so-prepared Re_(x)Mo_(1-x)O₃ powder is placed in the crucibles 24, which are presented here by a single compartment. The reaction with H₂S is carried out according to a procedure similar to the above described examples to prepare the solid solution. Once ready, the solid solution is placed in the reactor 12 (see FIG. 28) and the temperature is gradually raised to 650° C. under nitrogen atmosphere. The sample is heated at this temperature for 8 hr and then is left to cool down naturally under N₂ gas flow.

Example 6

Here instead of ReO₃ (Nb₂O₅) the more volatile halides, i.e. ReCl₄ and NbCl₄ have been used. FIG. 32 shows a reactor 12 used for this process, i.e. for the doping of IF-MoS₂ with Re or Nb which makes use of a supply of vaporized Re(Nb)Cl₄. The reactor 12 is associated with auxiliary chamber 14 or a heated bottle (see FIG. 33) for evaporation of the precursor. Here, a conduit tubing of NbCl₄ (ReCl₄) vapor is leading to the main reactor 12. The precursor NbCl₅ (mp 204.7° C., by 254° C.) is first heated in the auxiliary furnace 14 to a temperature of 250° C. (T4). The vapors of the heated precursor are mixed with N₂ (95%) H₂ (5%) gas (or pure N₂ gas) and are swept to the main reactor 12 (FIG. 32). The latter is generally similar to the reaction chambers 12 of FIG. 28, but here the precursor vapor flow is supplied in a direction towards that of the reaction gases.

Example 7

Here, the doping of preprepared INT-WS₂ (nanotubes) was made in a closed quartz ampoule as illustrated in FIG. 34. The ampoule 200 was placed in a two-zone furnace 22 allowing reaction materials to react for different periods of time (18 h and more) at different temperatures (from 800 to 600° C. in a relatively “cold zone” 40) with the growth zone (or “hot zone”) 50 at 950° C. (or less). The temperature gradient is intended to prevent back transport of the product. At the end of the reaction, the ampoule was cooled by water with ice, to quench the reaction at once. The total charge of the INT-WS₂ was maximum 250 mg. The stoichiometrically determined weight of the doping material (ReCl₄ and NbCl₄) was added to the load. In the present reaction iodine serves as the transport agent of the dopant atoms. The quartz ampoule 200 containing I₂ (less than 1 mg/cm³), ReO₃ (at the bottom of the ampoule) and INT-WS₂ (at the middle of the ampoule) was cooled with liquid nitrogen, evacuated to 10⁻⁵ Torr and sealed. The rhenium composition x was determined by EDS analyzer mounted on both the SEM and TEM. FIG. 35 shows a typical TEM image of the doped nanotubes. The nanotubes appear to be very similar to the undoped ones in terms of crystalline perfection, but they are somewhat more sensitive to beam damage.

In another related experiment, ReCl₄ was placed in the bottom of the ampoule (instead of ReO₃). This experiment led to improved doping characteristics of the nanotubes. The reason being that not only Re but also the chlorine served as an n-type dopant, due to the substation of the tungsten atoms in the nanotube lattice.

Example 8

Reference is made to FIG. 36 showing a modified fluidized bed reactor 10 with upper tubing (feeder) 60 which supplies ReCl₄ vapor to the main reactor 12 configured for undoped IF and INT nanoparticles. The auxiliary reactor whereby this mixture is prepared was described above with reference to FIG. 33. A feeder setup 70 supplies WS₂ nanotubes into the fluidized bed reactor 12 in a rate of 20-100 mg/minutes, and they undergo doping in the reactor. Alternatively, the INT-WS₂ powder can be placed on the filter and the gas fluidize it providing also reducing atmosphere which protects the nanotubes against oxidation.

The following is the characterization of the Re and Nb doped IF and INT. Low resistivity was measured and also very good tribological behavior was observed for the doped nanoparticles. Such nanoparticles provide very good dispersion in oil lubricants due to the extra charge on their surfaces. Furthermore, the charged nanoparticles get rid of static charge accumulated.

Example 9

In this example the above-described reactor of FIG. 36 is used to synthesize the Re (Nb) doped WS₂ nanotubes from their precursors. The precursors are WO₃ powder fed from above (or reduced WO_(2.9) oxide made of elongated shaped nanoparticles). A mixture of H₂S gas and a reducing (forming) gas containing N₂ (95%) and H₂ (5%) is flown from below to fluidize the nanoparticles. At the same time, a supply of ReCl₄(NbCl₄) is provided from an auxiliary system as described above with reference to FIG. 33. Re (Nb) doped WS₂ nanotubes are thus obtained.

Example 10

In this example, the Re doping process of pre-prepared INT-WS₂ and IF-WS₂ nanoparticles will be described. The doping procedure was performed using pre-prepared INT and IF nanoparticles by chemical vapor transport (CVT) method in sealed evacuated quartz ampoule (FIG. 34). Iodine was used as the transport agent of the dopant atoms to the nanoparticles. Two zone ampoule, doping material at the “hot” zone and NP at the “cold” zone, was designed to allow temperature gradient such that no back transport of the dopant atoms will occur.

Three batches of Re:INT-WS₂ and two batches of Re:IF-WS₂ were prepared. The details of each synthesis conditions are shown in Tables 7A and 7B. The stoichiometrically determined weight of the doping material was added to the load of the ampoule. A quartz ampoule containing less than 1 mg/cm³ halogen (iodine or chlorine) and the doping material at the “hot” zone, and INT or IF at the “cold” zone was evacuated to 10⁻⁵ Torr and sealed. The ampoule was placed in a two-zone furnace and allowed to react for desired periods of time at different temperatures. The maximum temperature in the cold zone can be up to 800° C. to avoid distortion of the NP. The temperature gradient is intended to prevent back transport of the dopant; therefore the temperature difference between the cold and the hot zone should be at least 150° C. and the distance between them should be at least 12 cm long. At the end of the reaction, the ampoule was cooled to quench the reaction at once. Notes for Re:INT-WS₂ (batch #3) and Re:IF-WS₂ (#2): The cold zone was at the bottom of the ampoule. Since ReCl₃ sublimating at ˜500° C., no transport agent was used.

The dopant composition was estimated using EDS (energy dispersive X-ray spectroscopy), and analyzed by SEM (scanning electron microscopy) and TEM (transmission electron microscope).

TABLE 7A Synthesis of Re: INT-WS₂ Transport Doping Weight added Synthesis Temperature Batch agent (TA) material (DM) TA DM duration Cold zone Hot zone #1 I₂ ReO₃ 1% of total ~5 wt % 22 h 800° C. 950° C. #2 ampoule load ~4 wt % 48 h #3 none ReCl₃ none ~2 wt % 48 h 350° C. 500° C.

TABLE 7B Synthesis of Re: IF-WS₂ Transport Doping Weight added Synthesis Temperature Batch agent (TA) material (DM) TA DM duration Cold zone Hot zone #1 I₂ ReCl₃ 1% of total ~2 wt % 96 h 350° C. 500° C. ampoule load #2 none ReCl₃ none 44 h 

The invention claimed is:
 1. A process for the manufacture of nanostructures each being of the formula A_(1-x)-B_(x)-chalcogenide wherein A is at least one of Mo, W, Re, Ti, Zr, Hf, Nb, Ta, Pt, Ru, Rh, In, Ga, WMo, or TiW; B is selected from Si, Nb, Ta, W, Mo, Sc, Y, La, Hf, Ir, Mn, Ru, Re, Os, V, Au, Rh, Pd, Cr, Co, Fe and Ni; and x≦0.3, provided that within said nanostructure A≠B; x is not zero; and B and B-chalcogenide are doped within the A_(1-x)-chalcogenide; the process comprising: providing A-oxide and B-oxide compositions, each in vapor phase, or in the solid phase under conditions permitting phase transformation into the vapor phase; or providing A_(1-z)-B_(z)-oxide composition (z≦0.01), in vapor phase, or in a solid phase under conditions permitting phase transformation into the vapor phase; and permitting said A-oxide and B-oxide vapors or said A_(1-z)-B_(z)-oxide vapors to flow together with a reducing agent carrying forming gas into a reaction chamber causing reduction of the A and B metals or transition metals and subsequent formation of said nanostructures.
 2. The process according to claim 1, wherein x is less than 0.01.
 3. A process for the manufacture of nanostructures each being of the formula A_(1-x)-B_(x)-chalcogenide wherein A is at least one of Mo, W, Re, Ti, Zr, Hf, Nb, Ta, Pt, Ru, Rh, In, Ga, WMo, or TiW; B is selected from Si, Nb, Ta, W, Mo, Sc, Y, La, Hf, Ir, Mn, Ru, Re, Os, V, Au, Rh, Pd, Cr, Co, Fe and Ni; and x<0.3, providing that within said nanostructure A≠B; x is not zero; and B and B-chalcogenide are doped within the A_(1-x)-chalcogenide; the process comprising: providing A-chalcogenide, in vapor phase; and permitting said A-chalcogenide vapor to flow together with a transport agent and a dopant precursor B-Y, wherein Y is selected from a halogen and a pnictide, in a reaction chamber under conditions that prevent or minimize back transport of atoms of said dopant, thereby causing doping of B into A-chalcogenide and formation of said nanostructures.
 4. The process of claim 1, wherein x is less than 0.005.
 5. The process of claim 1 wherein x is between 0.005 and 0.01.
 6. The process of claim 3, wherein x is less than 0.005.
 7. The process of claim 3 wherein x is between 0.005 and 0.01.
 8. The process of claim 3, wherein the transport agent is at least one halogen.
 9. The process of claim 8, wherein said halogen is chlorine, bromine or iodine.
 10. The process of claim 3, wherein the A-chalcogenide is provided in the solid phase under conditions permitting phase transformation into the vapor phase.
 11. The process according to claim 3, wherein the reaction chamber is a two zone chamber comprising a high temperature zone and a lower temperature zone, wherein the dopant precursor is fed into the high temperature zone and the A-chalcogenide is fed into the lower temperature zone.
 12. The process according to claim 11, wherein the maximal temperature of the high temperature zone is 800° C. and the high temperature zone and the lower temperature zone differ by a temperature exceeding 150° C.
 13. The process of claim 1, wherein said providing A-oxide and B-oxide compositions, in the solid phase under conditions permitting phase transformation into the vapor phase comprises: providing at least one concentric crucible comprising an inner and an outer portions; placing A-oxide composition in the outer portion of said crucible and B-oxide composition in the inner portion of said crucible, in the solid phase; and permitting A-oxide and B-oxide to transform into vapors.
 14. The process of claim 1, wherein said flowing of said A-oxide and B-oxide vapors together with said reducing agent carrying forming gas further comprises flowing of a chalcogenide carrying reacting gas, thereby causing occurrence of reduction of A and B, followed by a reaction with said chalcogenide carrying reacting gas resulting in the formation of said nanostructures.
 15. The process of claim 1, wherein said providing A_(1-x)-B_(x)-oxide composition in a solid phase under conditions permitting phase transformation into the vapor phase comprises placing said A_(1-x)-B_(x)-oxide composition in the solid phase in a crucible within said reaction chamber.
 16. The process of claim 1, wherein said providing A_(1-x)-B_(x)-oxide composition, in a solid phase under conditions permitting phase transformation into the vapor phase comprises: mixing powders of A-oxide and of B-oxide; treating said mixed powders, said treatment includes grinding or solvent addition or sonication or solvent drying or a combination thereof; placing said mixed powders in at least one crucible; inserting said at least one crucible into an auxiliary reactor; heating said crucibles under nitrogen; cooling down said crucibles, thus providing said A_(1-x)-B_(x)-oxide composition. 